Clean up leaked CSI snapshot for incomplete backup

[reasonerjt]

This commit makes sure when a backup is deleted the controller will delete the CSI snapshot even when the bakckup tarball is not uploaded.

fixes #8160

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Fixes #(issue)

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[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 59.39%. Comparing base (5b1738a) to head (1c37289).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/controller/backup_deletion_controller.go	70.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8637      +/-   ##
==========================================
+ Coverage   59.33%   59.39%   +0.06%     
==========================================
  Files         370      370              
  Lines       39932    39970      +38     
==========================================
+ Hits        23692    23742      +50     
+ Misses      14744    14736       -8     
+ Partials     1496     1492       -4     

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