Fix kubectl download URL in install script

[Warashi]

What this PR does:

This PR does the same as #5379 at the k8s plugin implementation.

Why we need it:

Not to use deprecated download URLs.

Which issue(s) this PR fixes:

Does this PR introduce a user-facing change?: No

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

[ffjlabo]

Thanks 🙏

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 25.72%. Comparing base (e276b89) to head (84df237).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5411      +/-   ##
==========================================
- Coverage   25.74%   25.72%   -0.03%     
==========================================
  Files         449      449              
  Lines       48452    48452              
==========================================
- Hits        12474    12464      -10     
- Misses      35005    35014       +9     
- Partials      973      974       +1     

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