chore: Remove release.gitlab.net instance references

Dat Tangrequested to merge
dattang/remove-release-gitlab-net into master

What does this MR do and why?

release.gitlab.net has been decommissioned, so all code and documentation referencing it should be removed.

Content

  • Delete GitlabReleaseClient class and its spec (was the dedicated API client for release.gitlab.net)
  • Delete Deployments::InstanceVersion class and its spec (wrapped GitlabReleaseClient)
  • Remove require lines for both deleted files from lib/release_tools.rb
  • Replace the release.gitlab.net Deploy section in templates/single_version_patch.md.erb with a note pointing to Release Environments and the connect-to-instance runbook

Closes gitlab-com/gl-infra/delivery#22039

Author Check-list

  • Has documentation been updated?

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