Use collectExistingBucket in GlobalOrdinalsStringTermsAggregator.WithHash.

Relates to #5955.
Run tests through forbidden-apis.
Use collectExistingBucket in GlobalOrdinalsSignificantTermsAggregator.WithHash.

Relates to #5955.
Added `reverse_nested` aggregation.

The `reverse_nested` aggregation allows to aggregate on properties outside of the nested scope of a `nested` aggregation.

Closes #5507
Fixed license headers.