etcdctl: Remove V2 API commands

[serathius]

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

Part of #12913
cc @ptabor

[serathius]

cc @ptabor @spzala @ahrtr

[ahrtr]

LGTM

[serathius]

cc @ptabor

[codecov-commenter]

Codecov Report

Merging #13643 (44161ed) into main (bf0b1a7) will increase coverage by 2.48%.
The diff coverage is 78.69%.

❗ Current head 44161ed differs from pull request most recent head 6da1c3a. Consider uploading reports for the commit 6da1c3a to get more accurate results

@@            Coverage Diff             @@
##             main   #13643      +/-   ##
==========================================
+ Coverage   72.57%   75.05%   +2.48%     
==========================================
  Files         465      444      -21     
  Lines       37856    36962     -894     
==========================================
+ Hits        27475    27743     +268     
+ Misses       8596     7405    -1191     
- Partials     1785     1814      +29     

Flag	Coverage Δ
all	75.05% <78.69%> (+2.48%)	⬆️

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

Impacted Files	Coverage Δ
server/config/config.go	79.51% <ø> (ø)
server/etcdmain/etcd.go	45.16% <0.00%> (-0.60%)	⬇️
server/etcdserver/bootstrap.go	68.79% <25.00%> (+0.67%)	⬆️
etcdctl/main.go	40.00% <33.33%> (+15.00%)	⬆️
server/embed/config.go	74.00% <68.42%> (+0.82%)	⬆️
server/etcdserver/api/v3discovery/discovery.go	80.00% <80.00%> (ø)
etcdctl/ctlv3/command/printer_json.go	77.35% <100.00%> (ø)
server/embed/etcd.go	74.80% <100.00%> (+0.60%)	⬆️
server/etcdmain/config.go	84.94% <100.00%> (+0.67%)	⬆️
server/auth/simple_token.go	80.00% <0.00%> (-8.47%)	⬇️
... and 24 more

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[ahrtr]

One more comment, we still support v2 discovery in 3.6. Should we keep the V2 commands, and delete them together the clientv2 & v2 discovery in future?

[ahrtr]

FYI discussion_r855723472

[ahrtr]

@serathius Please rebase this PR. I think it's OK to merge this PR for 3.6. The sever side V2 code has already been removed, so there is no reason to keep the V2 commands. If users need the v2 command, then they can get it from the previous releases, such as 3.5 or 3.4.