Don't allow users to change the value for --v2-deprecation

[ahrtr]

What would you like to be added?

part of #12913

If the v2store still contains custom content, 3.6 should panic on bootstrap. But if users change --v2-deprecation to V2Depr0NotYet, then it won't panic anymore even if v2store contains user data (custom content). It may lead to data inconsistency in such case.

To ensure a smoother upgrade from 3.5 to 3.6, we still keep the option, but ignore users' provided value; and print a warning if present.

Why is this needed?

To avoid potential data inconsistency due to users misconfiguration.

[ahrtr]

Note V2Depr0NotYet isn't a valid option any more starting from v3.6, see link below. But it still doesn't make sense to let users to configure the flag. We only need to proceed with deprecating v2store as scheduled; users don’t need to worry about opting in.

Actually there is no any difference between these valid options in 3.6, no matter what option users select, the behaviour is the same: panic on bootstrap if the v2store contains custom content. Note “write-only-drop-data” isn’t implemented in 3.6, and will be implemented in v3.7

etcd/server/etcdmain/config.go

Lines 98 to 101 in 7e04bd2

	v2deprecation: flags.NewSelectiveStringsValue(
	string(cconfig.V2Depr1WriteOnly),
	string(cconfig.V2Depr1WriteOnlyDrop),
	string(cconfig.V2Depr2Gone)),