s/plugin_new/plugin

There are historical reasons that the `plugin` command was `plugin_new`, now those are no longer applicable, we should remove the naming edge case from the project. This PR is based off of comments from #11176

ATP Railties
Remove redundant escapes from xml serialization test
Further clean-up of ActiveSupport::Callbacks

In #11195 I noticed a trailing comma in the docs, but I decided to
further clean it up.

What I have done:

* Clean up the trailing comma in the docs and some extra whitespace
  lines.
* Used `Array#extract` options to factor the repetitive pattern of
  `args.last.is_a(Hash) ? ...`
* Renamed the local var `config` to `options` in `define_callbacks`, as
  `options` seems to be the de facto name for the options objects.
* Renamed the local var `l` to `line` in `define_callback` (maybe it
  meant `lambda` in the context) as single `l` may look like `1` in some
  fonts.
Merge pull request #11211 from gsamokovarov/trailing-comma

Further clean-up of ActiveSupport::Callbacks
Merge pull request #11207 from vipulnsward/remove_escapes

Remove redundant escapes from xml serialization test
Removed deprecated method default_scopes?
Removed deprecated method scoped
Merge pull request #11203 from schneems/schneems/plugin_new-plugin

s/plugin_new/plugin
Merge pull request #11212 from neerajdotname/default_scopes_q_deprecated

Removed deprecated default_scopes?
Merge pull request #11213 from neerajdotname/scoped-deprecated

Removed deprecated scoped method

Conflicts:
	activerecord/CHANGELOG.md