Allow username to be used with multiple forges

[6543]

Followup of #5612 who missed that new we can have multiple users with same login name

[6543]

skip because we already have #5612 in the changelog :)

[woodpecker-bot]

Surge PR preview deployment was removed

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 21.23%. Comparing base (c77ca3e) to head (d7f2b16).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
server/forge/gitea/gitea.go	0.00%	3 Missing ⚠️
server/api/login.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5676      +/-   ##
==========================================
- Coverage   21.23%   21.23%   -0.01%     
==========================================
  Files         423      423              
  Lines       38279    38280       +1     
==========================================
  Hits         8130     8130              
- Misses      29395    29396       +1     
  Partials      754      754              

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[anbraten]

code lgtm, haven't test it