config: enable etcd api v2

[nolouch]

Signed-off-by: nolouch nolouch@gmail.com

What problem does this PR solve?

tiflash use v2 API.

What is changed and how it works?

enable v2 API.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #2091 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2091      +/-   ##
==========================================
+ Coverage   76.88%   76.96%   +0.07%     
==========================================
  Files         193      193              
  Lines       19419    19420       +1     
==========================================
+ Hits        14931    14946      +15     
+ Misses       3358     3350       -8     
+ Partials     1130     1124       -6

Impacted Files	Coverage Δ
server/config/config.go	83.45% <100%> (+0.88%)	⬆️
server/schedulers/shuffle_hot_region.go	64.58% <0%> (-5.21%)	⬇️
server/region_syncer/client.go	77.1% <0%> (-4.82%)	⬇️
server/schedulers/random_merge.go	60.71% <0%> (-3.58%)	⬇️
server/schedule/operator_controller.go	82.35% <0%> (+0.17%)	⬆️
server/grpc_service.go	57.88% <0%> (+0.21%)	⬆️
client/client.go	69% <0%> (+0.53%)	⬆️
server/server.go	82.89% <0%> (+0.56%)	⬆️
server/cluster/coordinator.go	78.39% <0%> (+0.83%)	⬆️
... and 5 more

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[rleungx]

LGTM

[lhy1024]

/merge

[sre-bot]

/run-all-tests