Add v1.6 release notes

[RainbowMango]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

This PR adds the release notes of release-1.6.

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 #3615 (1ba4a4f) into master (dff7a5b) will decrease coverage by 0.01%. The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master    #3615      +/-   ##
==========================================
- Coverage   55.22%   55.21%   -0.01%     
==========================================
  Files         221      221              
  Lines       20831    20831              
==========================================
- Hits        11504    11502       -2     
- Misses       8717     8719       +2     
  Partials      610      610              

Flag	Coverage Δ
unittests	55.21% <ø> (-0.01%)	:arrow_down:

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see 2 files with indirect coverage changes

[XiShanYongYe-Chang]

/lgtm

[karmada-bot]

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