Add Backup warning for inclusion of NS managed by ArgoCD

[shubham-pampattiwar]

Thank you for contributing to Velero!

Please add a summary of your change

This change adds a warning to velero backup operation if there are namespaces included in the backup which are managed by ArgoCD.

We’ve observed conflicts between Velero and ArgoCD in such environments and produce undesirable outcomes. This backup warning serves as a notification to ensure users are aware of the potential problems.

Does your change fix a particular issue?

Fix Related to #7905

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 64.28571% with 5 lines in your changes missing coverage. Please review.

Project coverage is 59.19%. Comparing base (42de654) to head (272b77e).

Files with missing lines	Patch %	Lines
pkg/backup/backup.go	64.28%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8257      +/-   ##
==========================================
- Coverage   59.21%   59.19%   -0.02%     
==========================================
  Files         367      367              
  Lines       30841    30855      +14     
==========================================
+ Hits        18262    18265       +3     
- Misses      11119    11128       +9     
- Partials     1460     1462       +2     

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[mateusoliveira43]

Should we check if label exist here instead of checking if its value is not an empty string?

[kaovilai]

That would probably be better.. as long as we know it is possible label only need key and empty value is ok

[shubham-pampattiwar]

The rationale here is the having an empty value for the ArgoCD label key would not make much sense, so this acts as a additional check.

[kaovilai]

While it may not make sense, we need to match whatever argocd logic is regarding namespace it manages. If it disallows empty value label then current code is fine.