Ensures that primary_key method will return nil when multi-pk

When table has a composite primary key, the `primary_key` method for
sqlite3 and postgresql was only returning the first field of the key.

Ensures that it will return nil instead, as AR dont support composite pks.
Merge pull request #18259 from arthurnn/return_nil_on_multi_pk

Ensures that primary_key method will return nil when multi-pk
 Update CHANGELOG entry for #18070 [ci skip]

 - Improved wording of CHANGELOG entry for https://github.com/arthurnn/rails/commit/5acd24bbeae0e9e5e81e87b5929e17f35527b2ea.
Make "Testing Views" separate section, move "Setup and Teardown" into
functional testing section and implement :show, :destroy, and :update tests.

[ci skip]
Merge pull request #18261 from prathamesh-sonpatki/update-changelog

 Update CHANGELOG entry for #18070 [ci skip]
Merge branch 'master' of github.com:rails/rails [ci skip]