Rename server.go to plugin.go in the k8s plugin's main package

[Warashi]

What this PR does:

Rename server.go to plugin.go in the k8s plugin's main package.

Why we need it:

for name consistency.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 26.14%. Comparing base (47a13c3) to head (33db5b4).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5463      +/-   ##
==========================================
+ Coverage   26.13%   26.14%   +0.01%     
==========================================
  Files         457      457              
  Lines       49107    49107              
==========================================
+ Hits        12835    12841       +6     
+ Misses      35250    35244       -6     
  Partials     1022     1022              

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