Pull the RelationMutationTests from cases/relation_test to cases/relation/mutation_test.
Deprecate the delegation of Array bang methods in ActiveRecord::Delegation

The primary means of returning results for Array bang methods is to modify
the array in-place. When you call these methods on a relation, that
array is created, modified, and then thrown away. Only the secondary
return value is exposed to the caller.

Removing this delegation is a straight-forward way to reduce user error
by forcing callers to first explicitly call #to_a in order to expose
the array to be acted on by the bang method.
Merge pull request #12129 from Empact/deprecate-array-bang-delegation

Deprecate the delegation of Array bang methods in ActiveRecord::Delegation

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/test/cases/relation_test.rb