[Feature Request] Remove `searchable_snapshot.extended_compatibility` experimental feature

[andrross]

Is your feature request related to a problem? Please describe

The "extended compatibility" experimental feature is intended to allow searchable snapshots to read older versions of OpenSearch snapshots that cannot be restored directly. While well-intended, this feature was introduced 2.5 years ago and never completed. There are additional complexities with enabling this feature for real and supporting long term.

Describe the solution you'd like

Remove this experimental feature.

Related component

Search:Remote Search

Describe alternatives you've considered

No response

Additional context

No response

[HyunsangHan]

Can I work on this? :)

[andrross]

Can I work on this? :)

@HyunsangHan Yes, by all means! I'll assign the issue to you.