Updated Golang version from v1.20.4 to v1.20.6

[parthn2]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
This PR updates the default Go version for CI and release bots to v1.20.6.s

Which issue(s) this PR fixes:
Fixes #3864

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Karmada(v1.7) is now built with Go1.20.6.

[karmada-bot]

Welcome @parthn2! It looks like this is your first PR to karmada-io/karmada 🎉

[codecov-commenter]

Codecov Report

Merging #3865 (5ffead8) into master (4488898) will increase coverage by 0.00%.
Report is 2 commits behind head on master.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master    #3865   +/-   ##
=======================================
  Coverage   54.95%   54.95%           
=======================================
  Files         227      227           
  Lines       21676    21676           
=======================================
+ Hits        11911    11912    +1     
+ Misses       9126     9125    -1     
  Partials      639      639           

Flag	Coverage Δ
unittests	54.95% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

see 1 file with indirect coverage changes

[RainbowMango]

/lgtm
/approve

Thanks @parthn2 and welcome to Karmada~

[karmada-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: RainbowMango

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