[Core] Remove v1 migrations

[rudolf]

We kept v1 migrations as a backup to workaround any issues we might see with v2 migrations. But since we're not actively testing v1 migrations on releases anymore we should aim to remove it as soon as possible to avoid regressions (such as #95617)

Since the only known problem has been solved in 7.12.1 we could keep it for 7.13 as an extra buffer, but then remove it in 7.14.0.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[rudolf]

Although the config value migrations.enableV2 will be deprecated in 7.12.1 (#96398) some users might not remember to remove this. It might be better to keep this configuration option but just ignore it's value than to fail to startup so that users who previously opted in to stay with V1 migrations will automatically start using v2 migrations?

[pgayvallet]

Is #91618 a blocker for that one?

[afharo]

We are now using V2 everywhere. There should be no blockers.

[joshdover]

Although the config value migrations.enableV2 will be deprecated in 7.12.1 (#96398) some users might not remember to remove this. It might be better to keep this configuration option but just ignore it's value than to fail to startup so that users who previously opted in to stay with V1 migrations will automatically start using v2 migrations?

+1 on this. This mirrors how we handle any of the xpack.code.* configs when that application was removed.

[pgayvallet]

We could just use an unused config deprecation, which would have the advantage to warn the user that this option is no longer required.