Docs: Remove UBI known limitations

What does this MR do?

Remove Registry UBI known limitation

Removes the known limitation that Registry does not provide UBI-based
images, as they have been available since GitLab 15.4:

```
$ skopeo list-tags docker://registry.gitlab.com/gitlab-org/build/cng/gitlab-container-registry | jq '.Tags' | grep 'v1\d\..*-ubi8' | head -n 1
  "v15.4.0-ubi8",
```

Remove GitLab Runner UBI known limitation

GitLab Runner has been publishing UBI-based images since v14.10:

```
$ skopeo list-tags docker://registry.gitlab.com/gitlab-org/gitlab-runner | jq '.Tags' | grep 'ubi-fips-v' | head -n 1
  "ubi-fips-v14.10.0",
```

Therefore, this commit removes that known limitation and updates the
UBI example values file to remove the portion that disables GitLab
Runner and replace it with an example of how to set the image tag
to point to a UBI image.

Changelog: other

Related issues

Closes #4315 (closed)

Testing

Confirm Registry

$ helm template gitlab . -f examples/ubi/values.yaml --set certmanager-issuer.email=a@b.com --show-only charts/registry/templates/deployment.yaml | yq '.spec.template.spec.containers[0].image'
registry.gitlab.com/gitlab-org/build/cng/gitlab-container-registry:v3.69.0-gitlab-ubi8

$ docker pull registry.gitlab.com/gitlab-org/build/cng/gitlab-container-registry:v3.69.0-gitlab-ubi8
v3.69.0-gitlab-ubi8: Pulling from gitlab-org/build/cng/gitlab-container-registry
2562ea2dcfe3: Pull complete
b81687e0b796: Pull complete
adcae8e09334: Pull complete
717f2ac48b2a: Pull complete
b5752656bb86: Pull complete
Digest: sha256:06dd2532a77fce1e01c79f14f7e60b5b1b67b2ad6a188cc04cf194e3c367b589
Status: Downloaded newer image for registry.gitlab.com/gitlab-org/build/cng/gitlab-container-registry:v3.69.0-gitlab-ubi8
registry.gitlab.com/gitlab-org/build/cng/gitlab-container-registry:v3.69.0-gitlab-ubi8

Confirm GitLab Runner

$ helm template gitlab . -f examples/ubi/values.yaml --set certmanager-issuer.email=a@b.com --show-only charts/gitlab-runner/templates/deployment.yaml | yq '.spec.template.spec.containers[0].image'
registry.gitlab.com/gitlab-org/gitlab-runner:ubi-fips-vX.Y.Z

$ docker pull registry.gitlab.com/gitlab-org/gitlab-runner:ubi-fips-v15.9.1
ubi-fips-v15.9.1: Pulling from gitlab-org/gitlab-runner
649e5534d134: Pull complete
fc15c2c6c6c1: Pull complete
4de37982e6d8: Pull complete
c9f2ed2d709c: Pull complete
Digest: sha256:1a0f431480c4fb8090bf6a65bc89fe69ec958d9d95ed6be12c0a5fc3399b2d81
Status: Downloaded newer image for registry.gitlab.com/gitlab-org/gitlab-runner:ubi-fips-v15.9.1
registry.gitlab.com/gitlab-org/gitlab-runner:ubi-fips-v15.9.1

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.