Add auto backport functionality to backport PRs for releases

[VachaShah]

Coming from meta issue opensearch-project/opensearch-plugins#108:

• Add auto-backport PR creation Github action to make sure all the PRs are backported for a particular release. Related documentation: https://github.com/opensearch-project/OpenSearch/blob/main/DEVELOPER_GUIDE.md.
• Add documentation for the same.

[joshuali925]

we won't be having more developments in this repo, since it's merged into https://github.com/opensearch-project/observability. closing