Closes #32581.
Closes #34028.
Closes #37343.

Use FileUpdateChecker for Migration::CheckPending 

Migration::CheckPending is a rack middleware normally used in
development to raise an exception if there pending migrations.

This commit replaces the existing caching, which avoided checking all
migrations if the newest migration (by version number) hadn't changed.
Instead, we now use FileUpdateChecker, which has two advantages: it can
detect new migrations which aren't the highest version, and it is
faster.

Update some gems to avoid warnings in ruby HEAD

```
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275: warning: method redefined; discarding old _context_get_included_files
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275: warning: method redefined; discarding old list_get_length
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275: warning: method redefined; discarding old list_get_value
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275: warning: method redefined; discarding old _make_data_context
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275: warning: method redefined; discarding old version
```

Were fixed in sassc 2.2.1.

```
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/json-2.1.0/lib/json/common.rb:156: warning: The last argument is nil, treating as empty keywords
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/json-2.1.0/lib/json/common.rb:156: warning: The last argument is used as the keyword parameter
```

Was fixed in 2.2.0.

```
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/i18n-1.6.0/lib/i18n/backend/base.rb:146: warning: The last argument is used as the keyword parameter
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/i18n-1.6.0/lib/i18n/backend/base.rb:84: warning: The last argument is used as the keyword parameter
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/i18n-1.6.0/lib/i18n.rb:179: warning: for `t' defined here
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/i18n-1.6.0/lib/i18n.rb:179: warning: for `translate' defined here
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/i18n-1.6.0/lib/i18n.rb:205: warning: The last argument is used as the keyword parameter
```

Was fixed by ruby-i18n/i18n@2920107fbfbf8778e5e197b7439d24c8a3720762 .

Rename load_paths.

[ci skip]

Add missing backticks for code conventions in contribution guide

While I was reading the "Contributing to Ruby on Rails" guide,
I noticed that `assert_not` was not being rendered as code inside
the "Follow the Coding Conventions" section.

[ci skip]

Don't modify relative ./ paths in BacktraceCleaner

Implicitly assert no exception is raised in block assertions

Update comment in controller generators

Fix comment for nested_records_changed_for_autosave? [ci skip]

In #33681 we stopped using "whitelist" in Rails in favor of
allowed/trusted. I found this because jbuilder is using old text from
2013 but noticed this still uses "white list". I think the new comment
is a bit clearer as well.

Document caveats in autosave association behaviour

Add SECURITY.md file, linking to security policy

Remove unused `require`s

`remove_method` were removed at 661fd98a and 737b718e

Fix IRB deprecation warning on tab-completion

Raise FrozenError for frozen objects when trying to write to a non-database-backed attribute

Use dynamic Rails version in plugin dummy apps

Avoid flunking tests on warnings in output

After having run the railties tests locally and running Rubocop, you'd
see lots of violations for the Rails apps generated in tmp folders.

Don't consider those violations as violating.

Docs: active job: Clarify default re: failure

Fixes https://github.com/rails/rails/pull/37113

fix(fork-tracker): Only patch fork if it's supported

Updated `ActionDispatch::Request.remote_ip=`

Some tests are currently wrapped with
  `if Process.respond_to?(:fork)`
but they're still executed on JRuby. The problem is that ForkTracker
unconditionally patches `#fork` so `Process` does indeed
`respond_to? :fork`.

Fix it by installing the patch conditionally with the same check.

Updated the setter to clear the value in the `@remote_ip` instance
variable before setting the header that the value is derived from in the
getter.

Fixed pluralization typo

This documents some of the subtler behaviour of autosave that was
established by eb22c248 and further refined by 5a3e34ed.