"Reject unsigned commits" push rule error message specifies GPG keys only
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### Summary
<!-- Summarize the bug encountered concisely. -->
When pushing an unsigned commit to a repository with the "Reject unsigned commits" push rule turned on, the error message is `Commit must be signed with a GPG key`.
This is misleading to users as they think that they have to set up GPG key commit signing specifically, but actually they could also set up SSH key or X.509 certificate commit signing to meet the push rule.
### Steps to reproduce
<!-- Describe how one can reproduce the issue - this is very important. Please use an ordered list. -->
1. Have a GitLab project that has the "Reject unsigned commits" push rule turned on, and a clone of it
2. Make an unsigned commit `git commit --no-gpg-sign -m 'test commit'`
3. Push to the GitLab repository `git push origin branchname`
### Example Project
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behavior, and link to it here in the bug report. If you are using an older version of GitLab, this
will also determine whether the bug is fixed in a more recent version. -->
I don't have access to reproduce this on GitLab.com because [it's a Premium/Ultimate feature](https://docs.gitlab.com/user/project/repository/signed_commits/#enforce-signed-commits-with-push-rules).
This is reproduced on the current GitLab Dedicated v19.2.4-ee.
### What is the current *bug* behavior?
<!-- Describe what actually happens. -->
The response is `Commit must be signed with a GPG key`
### What is the expected *correct* behavior?
<!-- Describe what you should see instead. -->
The response should be `Commit must be signed`, or `Commit must be signed with a GPG key, SSH key, or X.509 certificate`
### Relevant logs and/or screenshots
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as it's tough to read otherwise. -->
N/A
### Output of checks
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<!-- This bug happens on GitLab.com -->
<!-- and uncomment below if you have /label privileges -->
<!-- /label ~"reproduced on GitLab.com" -->
<!-- or follow up with an issue comment of `@gitlab-bot label ~"reproduced on GitLab.com"` if you do not -->
I don't have access to reproduce this on GitLab.com because [it's a Premium/Ultimate feature](https://docs.gitlab.com/user/project/repository/signed_commits/#enforce-signed-commits-with-push-rules).
This is reproduced on the current GitLab Dedicated v19.2.4-ee.
I can try to reproduce on latest but I would need to renew my local EE license first.
#### Results of GitLab environment info
<!-- Input any relevant GitLab environment information if needed. -->
<details>
<summary>Expand for output related to GitLab environment info</summary>
<pre>
(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)
(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)
</pre>
</details>
#### Results of GitLab application Check
<!-- Input any relevant GitLab application check information if needed. -->
<details>
<summary>Expand for output related to the GitLab application check</summary>
<pre>
(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:check SANITIZE=true`)
(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true`)
(we will only investigate if the tests are passing)
</pre>
</details>
### Possible fixes
<!-- If you can, link to the line of code that might be responsible for the problem. -->
https://gitlab.com/gitlab-org/gitlab/-/blob/99bbe4b00a4a352c0b319649d3838f2ebe19b281/ee/lib/ee/gitlab/checks/push_rules/commit_check.rb#L71
### Patch release information for backports
If the bug fix needs to be backported in a [patch release](https://handbook.gitlab.com/handbook/engineering/releases/patch-releases) to a version
under [the maintenance policy](https://docs.gitlab.com/policy/maintenance/), please follow the steps on the
[patch release runbook for GitLab engineers](https://gitlab.com/gitlab-org/release/docs/-/blob/master/general/patch/engineers.md).
Refer to the [internal "Release Information" dashboard](https://dashboards.gitlab.net/d/delivery-release_info/delivery3a-release-information?orgId=1)
for information about the next patch release, including the targeted versions, expected release date, and current status.
#### High-severity bug remediation
To remediate high-severity issues requiring an [internal release](https://handbook.gitlab.com/handbook/engineering/releases/internal-releases/) for single-tenant SaaS instances,
refer to the [internal release process for engineers](https://gitlab.com/gitlab-org/release/docs/-/blob/master/general/internal-releases/engineers.md?ref_type=heads).
<!-- If you don't have /label privileges, follow up with an issue comment of `@gitlab-bot label ~"type::bug"` -->
issue
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