transform zone to zones

[zishen]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

transform zone to zones

Which issue(s) this PR fixes:
part of #3952

Special notes for your reviewer:

@XiShanYongYe-Chang @whitewindmills

Does this PR introduce a user-facing change?:

none

[zishen]

@whitewindmills @RainbowMango

I cleared the zones when I set them because they can't exist at the same time.

I am not sure whether this change is reasonable. Could you please confirm it according to the business?

[codecov-commenter]

Codecov Report

Merging #3983 (45c9d5e) into master (19fbab5) will decrease coverage by 0.01%.
Report is 6 commits behind head on master.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #3983      +/-   ##
==========================================
- Coverage   54.77%   54.77%   -0.01%     
==========================================
  Files         232      232              
  Lines       22529    22541      +12     
==========================================
+ Hits        12341    12347       +6     
- Misses       9510     9516       +6     
  Partials      678      678              

Flag	Coverage Δ
unittests	54.77% <100.00%> (-0.01%)	⬇️

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

Files Changed	Coverage Δ
pkg/apis/cluster/mutation/mutation.go	98.18% <100.00%> (+0.06%)	⬆️

... and 1 file with indirect coverage changes

[whitewindmills]

I am not sure whether this change is reasonable. Could you please confirm it according to the business?

I think it's okay.

[RainbowMango]

I cleared the zones when I set them because they can't exist at the same time.

I guess you mean cleared the .spec.zone during migration.
Yes, I think we should do that.

[whitewindmills]

/lgtm

[RainbowMango]

/approve

[karmada-bot]

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