Fix balance-leader won't work when placement rule enabled (#2725)

[ti-srebot]

cherry-pick #2725 to release-4.0

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What problem does this PR solve?

When placement rule enabled, balance-leader won't work.

What is changed and how it works?

The region copy logic in ruleLeaderFilter's Target function is wrong. This request fix this bug.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release note

• Fix the bug that balance-leader won't work when placement rule enabled (imported from v4.0.3)

[Yisaer]

/run-all-tests

[codecov-commenter]

Codecov Report

Merging #2726 into release-4.0 will decrease coverage by 0.02%.
The diff coverage is 75.00%.

@@               Coverage Diff               @@
##           release-4.0    #2726      +/-   ##
===============================================
- Coverage        77.50%   77.47%   -0.03%     
===============================================
  Files              206      206              
  Lines            22490    22499       +9     
===============================================
+ Hits             17431    17432       +1     
- Misses            3758     3765       +7     
- Partials          1301     1302       +1     

Impacted Files	Coverage Δ
server/schedule/filter/filters.go	83.04% <40.00%> (-0.93%)	⬇️
pkg/mock/mockcluster/mockcluster.go	92.10% <100.00%> (+0.14%)	⬆️
server/schedulers/shuffle_hot_region.go	63.47% <0.00%> (-11.31%)	⬇️
server/tso/tso.go	78.94% <0.00%> (-4.61%)	⬇️
server/region_syncer/client.go	81.67% <0.00%> (-4.59%)	⬇️
pkg/etcdutil/etcdutil.go	88.40% <0.00%> (-2.90%)	⬇️
server/cluster/coordinator.go	70.02% <0.00%> (-1.92%)	⬇️
pkg/mock/mockhbstream/mockhbstream.go	89.23% <0.00%> (-1.54%)	⬇️
server/handler.go	50.76% <0.00%> (-0.44%)	⬇️
server/schedule/operator_controller.go	81.25% <0.00%> (-0.17%)	⬇️
... and 8 more

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[nolouch]

LGTM