[Remove] Deprecated Synced Flush API

[nknize]

This PR removes the deprecated synced flush API which was replaced by sequence number and retention lease mechanisms and no longer used in 2.0.0.

NOTE: this is not a blocker for Lucene 9 or 6.x deprecation removal

relates: #1674

[opensearch-ci-bot]

Can one of the admins verify this patch?

[opensearch-ci-bot]

❌   Gradle Check failure 57d27afd9aa7d5db8ef400e56e3751ae0efaaed6
Log 1573

Reports 1573

[andrross]

I went looking for any documentation for this API, and the only reference I found was in the security plugin. Will we need a follow up task to clean that up?

[opensearch-ci-bot]

❌   Gradle Check failure 19a7d655fd8e01778dd39113fbbbcf5d5de230c6
Log 1915

Reports 1915

[opensearch-ci-bot]

❌   Gradle Check failure 1b6cbfbe46ce6a594245c48ff09fe974af930ae2
Log 1930

Reports 1930

[opensearch-ci-bot]

✅   Gradle Check success c0b6125
Log 1968

Reports 1968

[nknize]

I went looking for any documentation for this API, and the only reference I found was in the security plugin. Will we need a follow up task to clean that up?

Looks like it's just an access permission for the _flush/synced endpoint? I think the security plugin should retain this so the endpoint can still be accessed and users receive the helpful endpoint removal warning (which we can fully remove in 3.0.0):

Synced flush was removed and a normal flush was performed instead. This transition will be removed in a future version.

[nknize]

This is ready for review.

[adnapibar]

LGTM