improve csr approver controller by replacing kubernetes client with client.Client

[rambohe-ch]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:
/kind bug
/kind documentation
/kind enhancement
/kind good-first-issue
/kind feature
/kind question
/kind design
/sig ai
/sig iot
/sig network
/sig storage

/kind enhancement

What this PR does / why we need it:

in controller-runtime v0.16, client.Client is supported to update csr approval. so remove original kubernetes client in csr approver controller and use the client.Client to replace it.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

other Note

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.1% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.01%. Comparing base (e42545e) to head (6687587).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2058      +/-   ##
==========================================
+ Coverage   55.53%   56.01%   +0.48%     
==========================================
  Files         186      186              
  Lines       18088    18082       -6     
==========================================
+ Hits        10045    10129      +84     
+ Misses       7013     6920      -93     
- Partials     1030     1033       +3     

Flag	Coverage Δ
unittests	56.01% <100.00%> (+0.48%)	⬆️

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[YTGhost]

/lgtm
/approve