[Cleanup] Clean up uses of LegacyESVersions

[msfroh]

Is your feature request related to a problem? Please describe

Now that we have released OpenSearch 3.0, we should probably start to clean up use of the LegacyESVersion class.

Describe the solution you'd like

I started to do this, thinking it would be quick and easy. It turns out to be more work than I thought, and should probably be broken down into some sub-tasks:

• Remove the upgrade-cli module. Its description is "A CLI tool for upgrading to OpenSearch 1.x from a supported Elasticsearch version." There are no supported Elasticsearch versions for 3.x.
• Clean up uses of LegacyESVersion.fromString. This is where we have some very old versions being referenced.
• Remove uses of constants from LegacyESVersion. In IndexMetadata, we can just assert unconditionally. The harder one is RemoteSnasphotDirectory. I strongly disagree with the extended compatibility feature on principle. That said, if we're going to let users shoot themselves in the foot anyway, IMO we should only let them do it going back to OpenSearch 1.0.
• Move idToVersion and stringToVersion maps from LegacyESVersion to Version.
• I think we can then remove LegacyESVersion.

Related component

Other

Describe alternatives you've considered

We could keep LegacyESVersion around through the 3.x line and remove it in time for 4.0?

Additional context

No response

[andrross]

@msfroh Can we carve out some "good first issues" here?

[msfroh]

@msfroh Can we carve out some "good first issues" here?

Sure thing!

I'll try out the "Create sub-issue" feature and tag them.

[msfroh]

The harder one is RemoteSnasphotDirectory. I strongly disagree with the extended compatibility feature on principle. That said, if we're going to let users shoot themselves in the foot anyway, IMO we should only let them do it going back to OpenSearch 1.0.

Fun fact -- I'm adding a new feature flag for something I'm working on and I added a unit test using reflection to confirm that all declared feature flags are properly registered. I had forgotten to register my new flag in FeatureFlagSettings, so I made sure the unit test would catch that and let me know. Imagine my surprise when it failed with the following message:

Feature flag opensearch.experimental.feature.searchable_snapshot.extended_compatibility.enabled is not registered in FeatureFlagSettings

So, it's not even possible to enable the extended compatibility feature. I think that means that we can safely scrap it.