Collapse where constraints to one where constraint

In order to remove duplication with joining arel where constraints with
`AND`, all constraints on `build_arel` are collapsed into one head node: `Arel::Nodes::And`

Closes: #11963
[ci skip] Improve readability of 4.3's NOTE in migration.md.
Merge pull request #12011 from jetthoughts/11963_fix_join_with_association_scope

Collapse where constraints to the Arel::Nodes::And node

Conflicts:
	activerecord/CHANGELOG.md
Merge pull request #12250 from JuanitoFatas/migration

[ci skip] Improve readability of 4.3's NOTE in migration.md.