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.

[downsrob]

I forked 2.x from main.
@prudhvigodithi Are we to bump main to 3.0 now or is that going to come in a separate campaign?

[downsrob]

ISM upgrade is currently blocked by notifications

* What went wrong:
Could not determine the dependencies of task ':integTest'.
> Can't get https://ci.opensearch.org/ci/dbc/distribution-build-opensearch/3.0.0/latest/linux/x64/tar/builds/opensearch/plugins/opensearch-notifications-3.0.0.0.zip to /workplace/downsrob/index-management/src/test/resources/notifications/opensearch-notifications-3.0.0.0.zip

[prudhvigodithi]

Hey @downsrob Checking OpenSearch core, main should point to 3.0.0, 2.x to 2.2.0.
Thank you
@dblock @bbarani @CEHENKLE

[downsrob]

We still need a 3.0.0 notifications zip to be published for ISM to upgrade.

[dblock]

The job-scheduler issue was fixed, see opensearch-project/job-scheduler#242, so notifications should be unblocked.

[dblock]

There should be nothing blocking this rn. To close this issue:

• main is building 3.0
• 2.x is building next 2.x, which is currently 2.4.0
• 2.3 is building next 2.3, which is 2.3.1
• manifests in https://github.com/opensearch-project/opensearch-build/tree/main/manifests include all 3