Update main and 2.x branches to comply with the OpenSearch plugin development standards

[ansjcy]

Is your feature request related to a problem?

We need to get release branches comply with the OpenSearch plugin development standards

What solution would you like?

After the plugin repo is set up with initial commits, we need to get main and releaes branches comply with the OpenSearch plugin development standards, more specifically, we need to set the OpenSearch version in main to 3.0.0-SNAPSHOT and cut the 2.x branch.

[ansjcy]

This issue is completed.