Drop gap_policy support for bucket_sort

[liketic]

Drop gap_policy support for agg bucket_sort.

Closes #36487

[elasticmachine]

Pinging @elastic/es-analytics-geo

[polyfractal]

Thanks @liketic!

After you opened the PR I realized there are some backwards compatibility concerns which I completely didn't think about in the original issue. I think we'll need to introduce a new flag (allow_gap_policy: true/false) to allow users to opt out of the gap_policy parameter, so that there is a migration path. Even though this is fixing a bug, it's introducing a breaking change which I didn't think about originally. Sorry :(

I think we'll need to do this:

• Introduce new allow_gap_policy flag in 6.7. Default to allow_gap_policy: true so that behavior doesn't change. Log a deprecation warning so the user knows it is going away.
• In 7.0, change default to allow_gap_policy: false, which throws an exception if the user attempts to use gap_policy on the bucket_sort agg. The user can change the flag to opt into the 6.x behavior if they want.
• In 8.0 we'll remove both allow_gap_policy and gap_policy parameters from bucket_sort

Which makes the change a lot more complicated than before. I understand if you don't want to deal with this, since the original issue looked much simpler and I marked it as a good adopt-me issue. Let me know if you want to continue, or we can close this PR and someone from the ES team can pick it back up.

Thanks, and sorry for the confusion!