Add maintenance history for backupRepository CRs

[Lyndon-Li]

Fix issue #7810, add maintenance history for backupRepository CRs

[Lyndon-Li]

[Breaking change]: LastMaintenanceTime should be used to record the completion time of a successful maintenance.
Previously, it is used to record the start time of a successful maintenance, which is not reasonable. E.g., I run a maintenance which takes more than 1 hour; since the default frequency for quick maintenance is 1 hour, the next maintenance will be launched immediately after the first one completes, but actually it is a wasting one as data has just been maintained.
Previously, the description in the CRD is ambiguous, it could mean the completion time or start time.

So in this PR, both the description and the value set to LastMaintenanceTime as changed.

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.

Project coverage is 59.20%. Comparing base (010fd1c) to head (92390e9).
Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/controller/backup_repository_controller.go	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8532      +/-   ##
==========================================
+ Coverage   59.18%   59.20%   +0.01%     
==========================================
  Files         369      369              
  Lines       39329    39427      +98     
==========================================
+ Hits        23276    23341      +65     
- Misses      14585    14617      +32     
- Partials     1468     1469       +1     

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