[INDEX] Ensure `index.version.created` is consistent

Today `index.version.created` depends on the version of the master
node in the cluster. This is potentially causing new features to be
expected on shards that didn't exist when the index was created.
There is no notion of `where was the shard allocated first` such that
`index.version.created` can't be reliably used as a feature flag.

With this change the `index.version.created` can be reliably used to
determin the smallest nodes version at the point in time when the index
was created. This means we can safely use certain features that would
for instance require reindeing and / or would not work if not the
entire index (all shards and segments) have been created with a certain
version or newer.

Closes #6660