Implement OpenSearch core branching strategy

[navneet1v]

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.