Update release notes on release-1.6 branch

[RainbowMango]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

This PR updates release notes on release-1.6 branch.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[codecov-commenter]

Codecov Report

Merging #3883 (48f0245) into release-1.6 (bf2358d) will decrease coverage by 0.01%.
The diff coverage is n/a.

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@@               Coverage Diff               @@
##           release-1.6    #3883      +/-   ##
===============================================
- Coverage        55.18%   55.18%   -0.01%     
===============================================
  Files              221      221              
  Lines            20840    20840              
===============================================
- Hits             11501    11500       -1     
- Misses            8727     8728       +1     
  Partials           612      612              

Flag	Coverage Δ
unittests	55.18% <ø> (-0.01%)	⬇️

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see 1 file with indirect coverage changes

[karmada-bot]

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