config: rename LeaderScheduleStrategy to LeaderSchedulePolicy

[rleungx]

What problem does this PR solve?

PR #2065 introduces a compatible problem. Consider that change is not released, we omit the compatible problem. But actually, there are still some tests that rely on the master branch. Closes #2072.

What is changed and how it works?

This PR rename LeaderScheduleStrategy to LeaderSchedulePolicy.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #2075 into master will decrease coverage by 0.13%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##           master    #2075      +/-   ##
==========================================
- Coverage    76.9%   76.76%   -0.14%     
==========================================
  Files         191      191              
  Lines       19293    19293              
==========================================
- Hits        14837    14810      -27     
- Misses       3346     3362      +16     
- Partials     1110     1121      +11

Impacted Files	Coverage Δ
server/schedule/selector/selector.go	95.78% <100%> (ø)	⬆️
server/core/store.go	78.62% <100%> (ø)	⬆️
server/config/config.go	89.47% <100%> (ø)	⬆️
server/schedulers/utils.go	88.82% <100%> (ø)	⬆️
server/cluster/cluster.go	81.31% <100%> (ø)	⬆️
pkg/mock/mockoption/mockoption.go	95.74% <100%> (ø)	⬆️
server/schedule/operator_controller.go	82.35% <100%> (+0.17%)	⬆️
server/schedule/operator/influence.go	80% <100%> (ø)	⬆️
server/schedule/checker/merge_checker.go	70.73% <100%> (ø)	⬆️
server/schedulers/balance_leader.go	90.84% <100%> (ø)	⬆️
... and 19 more

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[nolouch]

/merge

[sre-bot]

/run-all-tests