Implement OpenSearch core branching strategy

[prudhvigodithi]

Description

Ensure MAJOR_VERSION.x branch exists, the main branch acts as source of truth effectively working on 2 versions at the same time.

Related META issue

opensearch-project/opensearch-plugins#142

Current Behavior

Currently plugins follow a branching strategy where they work on main for the next development iteration, effectively working on 2 versions at the same time. This is not always true for all plugins, the release branch or branch pattern is not consistent, the lack of this standardization would limit multiple automation workflows and alignment with core repo. More details on META ISSUE

Proposed solution

Follow OpenSearch core branching. Create 1.x and 2.x branches, do not create 2.0 as a branch of main, instead create main -> 2.x -> 2.0. Maintain working CI for 3 releases at any given time.

[prudhvigodithi]

Hey @qreshi can you please post an update and make the necessary changes to align with core branching strategy.
Thank you
@dblock @CEHENKLE @bbarani

[qreshi]

I was blocked on Common Utils before. I'll proceed with updating Notifications now.

[qreshi]

Upgrading main to 3.0 is currently blocked on opensearch-project/OpenSearch#3615

[qreshi]

Since the issue linked above is still ongoing and we can't upgrade to 3.0 on main, I'll update the version tag for this issue for tracking. I'd like to make sure main is on the most up-to-date development version before closing this issue out.