Skip Spring App Generator tests on JRuby

Spring makes extensive use of Process.fork, so won't be able to provide
JRuby Support, hence skip these tests on JRuby.
Merge pull request #13687 from gaurish/spring-has-no-love-for-jruby

Skip Spring App Generator tests on JRuby
doc proc/lambda arg on inclusion validation. Closes #13689. [ci skip]
highlight http://localhost:3000 in README.md. Closes #13643. [ci skip]

This has no effect on the API docs. The main API page is taken from:
    railties/RDOC_MAIN.rdoc
fix bug in becomes! when changing from base to subclass. Closes #13272.
standardize on jruby_skip & rbx_skip

This Adds helpers(jruby_skip & rbx_skip). In Future, Plan is to use
these helpers instead of calls directly to
RUBY_ENGINE/RbConfig/JRUBY_VERSION
Merge pull request #13693 from gaurish/typo

[JRuby] Fix incorrect RUBY_ENGINE value
Updated Changelog to reflect removal of :dependent => :restrict

Based on https://github.com/rails/rails/issues/2502, the option has
been removed.
Merge pull request #13699 from rystraum/dependent-restrict-removal

Updated Changelog to reflect removal of :dependent => :restrict
Only lookup `config.log_level` for stdlib `::Logger`. Closes #11665.

This prevents Rails from assigning meaningless log levels to third
party loggers like log4r. If `Rails.logger` is not `kind_of?(::Logger)`
we simply assign the `config.log_level` as is.

This bug was introduced by #11665.
removed extra comma [ci skip]
Merge pull request #13702 from kuldeepaggarwal/docs-fix

removed extra comma [ci skip]
Favor canonical environment variables for secrets

Prefixing an environment variable with `RAILS_` should be used when there is otherwise a conflict, such as `RAILS_ENV` or if it is being used for a very Rails specific value.

If we remove the prefix of `RAILS_` in the case of `RAILS_SECRET_KEY_BASE` then we can push for a pseudo standard among other frameworks that will accept a common environment key `SECRET_KEY_BASE` to keep your app secure. This is beneficial for containerized deployments such as docker, Heroku, etc. So that the container need to know one less thing about your app (it can just set it no-matter what language or framework you are using). 

This change also allows the defaults to be consistent with the way the secret key is accessed so `secrets.secret_key_base` is aliased to `SECRET_KEY_BASE` instead of `RAILS_SECRET_KEY_BASE`.
Merge pull request #13703 from schneems/schneems/rail-secrets-key

Favor canonical environment variables for secrets