fix: support pdb in kube 1.20

[Rory-Z]

Fix #1045

[codecov]

Codecov Report

Attention: Patch coverage is 27.77778% with 39 lines in your changes are missing coverage. Please review.

Project coverage is 76.56%. Comparing base (2cf7e02) to head (dd8307c).

Files	Patch %	Lines
controllers/apps/v2beta1/add_pdb.go	27.77%	38 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1047      +/-   ##
==========================================
- Coverage   76.78%   76.56%   -0.23%     
==========================================
  Files          69       69              
  Lines        6202     6250      +48     
==========================================
+ Hits         4762     4785      +23     
- Misses       1202     1228      +26     
+ Partials      238      237       -1     

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