[ISSUE-524][operator] Upgrading rss could also be deleted

[advancedxy]

What changes were proposed in this pull request?

1. soft the constraint that upgrading rss object cannot be deleted

Why are the changes needed?

more flexibility.
This fixes #524

Does this PR introduce any user-facing change?

For RSS cluster admin, they can delete a upgrading rss cluster.

How was this patch tested?

Manually verified and an added UT

[advancedxy]

ping @wangao1236

[advancedxy]

@wangao1236 please take a look. This should be ready for merge.

[codecov-commenter]

Codecov Report

Merging #531 (a7263f1) into master (ebbe2db) will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #531      +/-   ##
============================================
+ Coverage     60.21%   60.25%   +0.03%     
  Complexity     1784     1784              
============================================
  Files           205      205              
  Lines         11557    11558       +1     
  Branches       1042     1042              
============================================
+ Hits           6959     6964       +5     
+ Misses         4190     4187       -3     
+ Partials        408      407       -1     

Impacted Files	Coverage Δ
...bernetes/operator/pkg/controller/controller/rss.go	33.22% <0.00%> (+0.74%)	⬆️
...y/kubernetes/operator/pkg/webhook/inspector/rss.go	47.70% <ø> (ø)

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