[ML] make bucket_correlation aggregation generally available (#88655)

Originally released in 7.14, bucket_correlation is now generally available.

docs/changelog/88655.yaml

@@ -0,0 +1,5 @@
+pr: 88655
+summary: Make `bucket_correlation` aggregation generally available
+area: Machine Learning
+type: feature
+issues: []

docs/reference/aggregations/pipeline/bucket-correlation-aggregation.asciidoc

@@ -5,8 +5,6 @@
 <titleabbrev>Bucket correlation</titleabbrev>
 ++++
 
-experimental::[]
-
 A sibling pipeline aggregation which executes a correlation function on the
 configured sibling multi-bucket aggregation.