enable automatic backport to 2.x

[rursprung]

since we now have a 2.x branch here (since #69) it'd be good if the automated backport bot would be set up the same way as for the other opensearch repos (i'm not familiar with the details - i just know that it exists 😄) to ensure that 2.x is kept in sync where relevant.

until this is the case cherry-picks will have to be done manually.

[dblock]

@rursprung You can just copy the backport workflow from maybe https://github.com/opensearch-project/opensearch-java/blob/main/.github/workflows/backport.yml

#83