[Proposal] Enable searchable snapshots to read legacy versions

[kartg]

Is your feature request related to a problem? Please describe.
Expand searchable snapshots to support read-only access to snapshots written by legacy versions. This would be useful to customers storing data for long periods of time (and thereby across major version upgrades) for log analysis or audit use-cases.

Describe the solution you'd like
Leverage Lucene 9's ability to read older versions to expand backwards compatibility for searchable snapshots to allow reading older snapshots.

Describe alternatives you've considered
N/A

Additional context
N/A

[dblock]

What is considered "legacy" @kartg in 1.x, 2.x, and 3.x?

[kartg]

What is considered "legacy" @kartg in 1.x, 2.x, and 3.x?

@dblock IMO a "legacy" version is any version older than OpenSearch's standard backwards compatibility guarantee (one prior major version). So:

• For 3.x, OpenSearch 1.x (or ElasticSearch 7.10.2) and lower versions are considered legacy
• For 2.x -> ElasticSearch 6.x and lower
• For 1.x -> ElasticSearch 5.x and lower

I think these should also have a lower bound, but I don't have a value in mind at this time. For example, the implementation in my fork does not currently work for ElasticSearch 5.x snapshots, but I believe I can get it to function.

[kartg]

PR to 2.x - #5429

[kartg]

PR to main - #5812