Don't run maintenance on the ReadOnly BackupRepositories.

[blackpiglet]

Thank you for contributing to Velero!

Please add a summary of your change

Does your change fix a particular issue?

Fixes #8238

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file (make new-changelog) or comment /kind changelog-not-required on this PR.
• Updated the corresponding documentation in site/content/docs/main.

[codecov]

Codecov Report

Attention: Patch coverage is 43.75000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 59.40%. Comparing base (79707aa) to head (fa156c3).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/controller/backup_repository_controller.go	43.75%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8681      +/-   ##
==========================================
+ Coverage   59.39%   59.40%   +0.01%     
==========================================
  Files         370      370              
  Lines       39988    40050      +62     
==========================================
+ Hits        23749    23792      +43     
- Misses      14746    14766      +20     
+ Partials     1493     1492       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.