Fix AC::TemplateAssertions instance variables not resetting.

Fixes https://github.com/rails/rails/issues/16119.
Remove the side-effects of validates_presence_of.
Merge pull request #16248 from zuhao/refactor_activerecord_presence_validation_test

Remove the side-effects of validates_presence_of.
docs, add example for running a single test against core adapters.

[ci skip]
Remove length limit that results failure in other tests.
Merge pull request #16250 from zuhao/refactor_activerecord_migration_test

Remove length limit that results failure in other tests.
build fix, remove not null constraint.

The fixtures are still in play, adding a new column without a default and `null: true`
is not possible. This reverts back to leaking global state, as our `schema.rb` adds
the `null: false` constraint on this field.

A future solution would be to make the `migration_test.rb` run independent of fixture tables.
This way we can simply drop the state after test execution, without worrying about side effects.

/cc @zuhao
Make sure assertion passes for postgresql.
Remove redundant that already happened in teardown.
Merge pull request #16254 from zuhao/refactor_activerecord_attribute_decorators_test

Remove redundant reset that already happened in teardown.
Merge pull request #16253 from zuhao/refactor_activerecord_nested_through_associations_test

Make sure assertion passes for postgresql.
Allow to specify a type for foreign key column in migrations

[Andrey Novikov & Łukasz Sarnacki]
Merge pull request #16231 from Envek/type_in_references

* Allow to specify a type for foreign key column in migrations
* unified the docs
* some cleanup in CHANGELOG
Fixes #16255 [ci skip]
Merge pull request #16256 from akshay-vishnoi/doc-change

Fixes #16255 [ci skip]
Merge pull request #16234 from tgxworld/fix_template_assertion_for_integration_test

Fix AC::TemplateAssertions instance variables not resetting.
`create` return an active record object with erros instead of false when validation fails. [skip ci]
Merge pull request #16237 from tonytonyjan/patch

`create` method returns an active record object with errors instead of false when validation fails. [skip ci]
specify N=0 to prevent parallel tests (just run tests on the main process)
Use PROCESS_COUNT to define the number of parallel executors