Fix crashing on circular left join references with scoping

`gmail` --> `Gmail`

[ci skip]

[ci skip]fix wrong variable name in docs

Bump RuboCop to 0.49.0 and fix offences

Update Rails API Application guide to match current code [ci skip]

Update information on using Gmail when 2FA is in use [ci skip]

Follow up of #25702.

[ci skip]fix wrong method name in docs

Fix select tag helper used with Enumerable choices

Make helper methods in tests to private

Both reference id and type should be `NOT NULL` if `null: false` is specified

Set `Capybara.app_host` through `host!`

Adding Foreing Key - Don't worry about building the identifier if name is already present.

Remove a duplicate test of inverse_associations_test in AR

Remove unused `JoinPart#name`

Remove rubygems version lock

Remove unused `left_joins_values` generation

Refactor making join constraints

More friendly exception in nested attributes

This method was only used in the Rails tests and not by other methods in
the Rails simulator. Because it's a no-doc'd class it should be safe to
remove without deprecation.

The only difference between `make_inner_joins` and
`make_left_outer_joins` is the `join_type`.

This was added at #22125 but `left_joins_values` is never used.

This version lock added by 3d890b66.
But original issue is fixed with bundler 1.15.

Ref: https://github.com/rubygems/rubygems/issues/1911#issuecomment-300148516

Update test link in ActionView javascripts README.md.

[ci skip]

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Simplify handling of prerequisites in Getting Started guide [ci skip]

Merge pull request #29072 from dixpac/dixpac/add_documentation_for_validate_and_callback_order_of_execution

Improving docs for callbacks execution order [ci skip]

Telling people about prerequisites, and then telling them a page later
how to check and install those prerequisites, is confusing. This commit
removes the list and just handles the software installation in one
place.

Fixes #28565

When define callbacks latest definition on the same callback/method
overwrites previous ones.