5_1_release_notes include more removals [ci skip]

Removed unnecessary semicolons

Add load hook for `ActionDispatch::SystemTestCase`

Remove mention of X-Post-Data-Format header [ci skip]

See: https://github.com/teamcapybara/capybara/blob/7d693f068c44f6a460336da70fb6e9e5f94f3db9/lib/capybara.rb#L450

This is useful to extend `SystemTestCase`.
Also, since other test classes already have load hooks, should also be
in `SystemTestCase`.

Ref: 0510208d

Support for this header was removed when `actionpack-xml_parser` was
extracted, and has since been dropped from the gem.

- It is present in Rails 5.1.3 already.
- Backport commit https://github.com/rails/rails/commit/9efa0b70a638dcfdc05b30ab530f89f2847bb9c2

```
% git grep -n trough
activerecord/test/cases/associations/has_many_through_associations_test.rb:1253:  def test_has_many_trough_with_scope_that_has_joined_same_table_with_parent_relation
```

delegate scope_for on PolymorphicReflection

fix initial count

Allow passing a Proc or Symbol as an argument to length validator values

This brings the Length validator in line with the Numericality
validator, which currently supports Proc & Symbol arguments

Remove mention about Evented Redis [ci skip]

Evented Redis adapter was removed in 48766e32.

remove incorrect comment about serializable transactions [ci skip]

using serializable isolation would prevent the duplicate insert as done in the example from happening

Remove unused `secrets.yml` template

Move `extensions` to `PostgreSQL::SchemaDumper`

- Backport commit: https://github.com/rails/rails/commit/7122a2cdc3634e170129f8b6cabd1e8fbed13c3d

Wrap accepted nested attributes params

to properly wrap all attributes, including those which are nested.

and abstract `SchemaDumper#extensions` is now an empty method.

Since #30337, every database adapter has its own `SchemaDumper`.
`extensions` are only supported by PostgreSQL database and postgresql database adapter.

Save index order :desc to schema.rb (sqlite). Fixes #30902

[ci skip]Add space before closing curly brace

Does not include disabled element in params

Change `VENDOR_PATH` to `APP_ROOT` in `bin/yarn`

This variable was initially used to hold the vendor directory.
https://github.com/rails/rails/commit/3dac36b

Therefore, the variable name `VENDOR_PATH` was appropriate.
However, `package.json` is now placed in the root of the project.
https://github.com/rails/rails/commit/8e9e943

Therefore, like other bin scripts, I think the variable name `APP_ROOT`
is appropriate.
https://github.com/rails/rails/blob/2c845f6b03ddf2aa233b00385d24d769a4a34fa6/railties/lib/rails/generators/rails/app/templates/bin/setup.tt#L5
https://github.com/rails/rails/blob/2c845f6b03ddf2aa233b00385d24d769a4a34fa6/railties/lib/rails/generators/rails/app/templates/bin/update.tt#L5

`supports_extensions?` return always true since PostgreSQL 9.1