Move ErrorTrackingOpenAPI gem from vendor/gems to gems/

What does this MR do and why?

This MR moves ErrorTrackingOpenAPI gem from vendor/gems to gems/

• Adjust path in Gemfile and doc/
• Trigger a child pipeline in gems/ only if .gitlab-ci.yml exists
  • This is useful when neither nor .rubocop.yml nor specs exists.

Implements https://docs.gitlab.com/ee/development/gems.html#in-the-same-repo.

No RuboCop and no RSpec?

Because this gem is generated by rake gems:error_tracking_open_api:generate this gem is missing:

• No RuboCop configuration because the generated code won't apply standards (gitlab-styles)
• No RSpec examples because the generated specs are stubs and don't test anything

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.