Improve error message for `job_activity_limit_exceeded`

Summary

When the CI limit job_activity_limit is hit we drop a pipeline with an error Pipeline job activity limit exceeded!.

This error occurs if too many jobs/pipelines are being created in a short period of time.

As a consequence, we drop the pipeline with that error. Users will need to wait until the volume of jobs in active pipelines goes below the limit (today it's 500 jobs for projects on Free tier).

I think this issue #221245 will help here because it introduces a category for pipeline messages (errors or warnings) and the category can have a documentation link associated to it. This will improve the error messages across the board because it will display a link to the related documentation. This way we can improve the docs rather than be too verbose in the message.

Steps to reproduce

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Example Project

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What is the current bug behavior?

(What actually happens)

What is the expected correct behavior?

(What you should see instead)

Relevant logs and/or screenshots

Screenshot

Output of checks

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Results of GitLab environment info

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`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`)

Results of GitLab application Check

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(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)

Possible fixes

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