server/schedule: use ChangePeerV2 to demote single voter directly

[w41ter]

What problem does this PR solve?

ref #4444.
ref tikv/tikv#11463.

What is changed and how it works?

Change the field ChangePeer to ChangePeerV2 if demoting a voter directlry. This can avoid breaking compatibility.

Check List

Tests

• No code

Related changes

• Need to cherry-pick to the release branch

Release note

Fix the problem that demoting a follower to learner isn't working.

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[w41ter]

In addition to this PR, please also help me to review : pingcap/kvproto#839.

[codecov]

Codecov Report

Merging #4445 (0b38d02) into master (3c626f2) will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #4445      +/-   ##
==========================================
+ Coverage   74.78%   74.82%   +0.03%     
==========================================
  Files         264      264              
  Lines       27789    27798       +9     
==========================================
+ Hits        20783    20799      +16     
- Misses       5140     5145       +5     
+ Partials     1866     1854      -12     

Flag	Coverage Δ
unittests	74.82% <0.00%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
server/schedule/operator/step.go	72.41% <0.00%> (-2.32%)	⬇️
server/schedule/operator_controller.go	83.50% <0.00%> (-1.23%)	⬇️
server/schedulers/random_merge.go	60.00% <0.00%> (-3.34%)	⬇️
server/member/member.go	63.10% <0.00%> (-2.14%)	⬇️
server/tso/allocator_manager.go	64.50% <0.00%> (-1.50%)	⬇️
server/grpc_service.go	51.78% <0.00%> (+0.19%)	⬆️
client/client.go	72.78% <0.00%> (+0.21%)	⬆️
server/config/persist_options.go	92.42% <0.00%> (+0.75%)	⬆️
server/server.go	72.35% <0.00%> (+0.88%)	⬆️
... and 6 more

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[w41ter]

/test

[w41ter]

/run-all-tests

[HunDunDM]

Since ChangePeerV2 is used, useJointConsensus == true is required. This shows that the judgment condition of useJointConsensus and the logic of buildStepsWithJointConsensus need to be changed. At the same time, DemoteFollower will not be used in buildStepsWithoutJointConsensus, it must be split into RemovePeer and AddLearner.

[HunDunDM]

We need a new way of implementation.

[HunDunDM]

/hold

[nolouch]

ptal @HunDunDM

[bufferflies]

how about add unit test?

[HunDunDM]

#4444 is implemented in #4534, maybe we can close this PR? If you have any questions about #4534, please ask.