Switch to validating v3 when v2 and v3 are synchronized

[serathius]

This PR is not very useful, but let's merge it to clarify how validation should look like as long as we bootstrap from v2 store

Alternatives proposed before:

• Validate conf change using v3 #16084
• Stop writing v2store and validate confChange against v3store instead of v2store #17019

[k8s-ci-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[serathius]

cc @ahrtr @fuweid @chaochn47 @siyuanfoundation

[fuweid]

LGTM

[ahrtr]

Alternatives proposed before:

• Validate conf change using v3 #16084
• Stop writing v2store and validate confChange against v3store instead of v2store #17019

To be clearer, It isn't an alternative solution. This PR is just a solution on top of #16084 with a further fix (thx for that). I would raise a comment on 16084 and honor the original author's work (@geetasg, thanks for your contribution & patience).