upgrade rsa key size from 2048 to 3072

[chaosi-zju]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

According to NSA and ANSSI, RSA with 3072 bit-modulus is the minimum to protect up to TOP SECRET.

We should not be in the red line of cryptography. This security by default will bring the needed protection for users.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.31%. Comparing base (fcb46bb) to head (dee30c8).
Report is 2 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4955   +/-   ##
=======================================
  Coverage   53.31%   53.31%           
=======================================
  Files         252      252           
  Lines       20485    20485           
=======================================
  Hits        10921    10921           
  Misses       8842     8842           
  Partials      722      722           

Flag	Coverage Δ
unittests	53.31% <ø> (ø)

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[chaosi-zju]

/retest

[RainbowMango]

@zhzhuang-zju @yanfeng1992 Would you like to take a look?

[yanfeng1992]

/lgtm

[RainbowMango]

/approve

[karmada-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: RainbowMango

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[zhzhuang-zju]

/lgtm