Add changelog of release-1.8

[RainbowMango]

What type of PR is this?

/kind documentation

What this PR does / why we need it:
This PR adds the release notes of release-1.8.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
This release includes 230+ commits from 30+ contributors, see the full list of commits here.

Does this PR introduce a user-facing change?:

NONE

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7032767) 51.99% compared to head (403fa38) 51.99%.
Report is 4 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4336   +/-   ##
=======================================
  Coverage   51.99%   51.99%           
=======================================
  Files         242      242           
  Lines       23990    23990           
=======================================
  Hits        12474    12474           
- Misses      10835    10836    +1     
+ Partials      681      680    -1     

Flag	Coverage Δ
unittests	51.99% <ø> (ø)

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[karmada-bot]

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