Add missing http status codes [ci skip]

Ref: https://github.com/rack/rack/commit/5401f776f660b1f8d0e0650ba78478d7488eff75,
https://github.com/rack/rack/commit/73e08279d4433ca66cf22157a40dba562629031a

Using require_relative in the Rails codebase

Remove redundant `assert_respond_to`

It is covered by following assertion.

Don't wrap parameters if query parameter exists

Fix removed version 5.2 to 6.0 in the deprecation message

Move test related to `tmp:clear` task to `tmp_test.rb`

Because the deprecation message is not yet released.

Let's say you have a nested transaction and both records are saved.
Before the outer transaction closes, a rollback is performed. Previously
the record in the outer transaction would get marked as not persisted
but the inner transaction would get persisted.

```ruby
Post.transaction do
  post_one.save # will get rolled back

  Post.transaction(requires_new: true) do
    post_two.save # incorrectly remains marked as persisted
  end

  raise ActiveRecord::Rollback
end
```

To fix this the PR changes transaction handling to have the child
transaction ask the parent how the records should be marked. When
there are child transactions, it will always be a SavpointTransaction
because the stack isn't empty. From there we pass the parent_transaction
to the child SavepointTransaction where we add the children to the parent
so the parent can mark the inner transaction as rolledback and thus mark
the record as not persisted.

`update_attributes_from_transaction_state` uses the `completed?` check to
correctly mark all the transactions as rolledback and the inner record as
not persisted.

```ruby
Post.transaction do
  post_one.save # will get rolled back

  Post.transaction(requires_new: true) do
    post_two.save # with new behavior, correctly marked as not persisted
    on rollback
  end

  raise ActiveRecord::Rollback
end
```

Fixes #29320

`set_state` was directly setting the transaction state instance
variable. It's better to set the state via specific methods (`rollback!`
and `commit!` respectively.

While undocumented and untested, it's possible someone is using
`set_state` in their app or gem so I've added a deprecation notice to
it.

No where in the app do we use `nullify!` but I wanted to keep existing
behavior while replacing the method with a better pattern.

Warning http verb method call in SystemTestCase

This reverts commit 3420a145, reversing
changes made to afb66a5a.

Enforce frozen string in Rubocop

Make ActiveSupport frozen-string-literal friendly.

Add srcset option to image_tag helper

Clear screenshots files in `tmp:clear` task

Add source code and changelog link to railties.gemspec

Should be clear `@association_ids` when joined newly associated record

Don't cache queries for schema statements

Properly register "custom" URL helpers as named helpers.

Fix automatic inverse for polymorphic interfaces

Ensure `false` is preserved in attr serialization