Enforce exact matching for GitLab groups

[pat-s]

fix #2922

Until now, GL groups were matched partially. This caused an issue in the following case:

1. In a new instance with only subgroup repos enabled
2. Enable a non-subgroup repo
3. Partial match for the first owner/subgroup of the existing orgs
4. Attempt to (re)create owner/subgroup although it already exists -> error

By enforcing an exact match, the query returns nil for a non-existing top-level group and will attempt to (correctly) create a new WP org for it, letting the overall repo add operation succeed.

Fix has already been tested to work.

I don't think there should be any potential downsides when switching to exact matching.

Besides, I also updated the error message wording I encountered.

Should be backported.

[woodpecker-bot]

Deployment of preview was torn down

[6543]

that should be backported, wana do it or should I cherry pick?

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.

Project coverage is 28.22%. Comparing base (896e550) to head (50717a3).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
server/forge/gitlab/gitlab.go	0.00%	9 Missing ⚠️
server/api/repo.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4473      +/-   ##
==========================================
+ Coverage   28.20%   28.22%   +0.01%     
==========================================
  Files         385      385              
  Lines       27847    27852       +5     
==========================================
+ Hits         7855     7860       +5     
  Misses      19301    19301              
  Partials      691      691              

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[6543]

Backport: #4474