Upgrade buckler plugin to be compatible with the last stable Elasticsearch 7.10.0

As per this documentation about remote elasticsearch requirements for bitbucket, the latest supported buckler plugin (2.1.3) works with elasticsearch is 7.5.2 .

Also, I searched on your maven repo, and i found that you had buckler (2.1.4) which seems to work with elasticsearch 7.9.3 . However, it's not listed in the documentation mentioned previously.

Could you update the compatibility matrix in the official documentation ? 

What's the deadline to release buckler plugin compatible with Elasticsearch 7.10.0

Note, that Elasticsearch 8.0.0 is on the way.