Added missing changelog entry for new guide added for Multiple Databases with Active Record(#36389) [ci skip]

* Update RAILS_VERSION
* Bundle
* rake update_versions
* rake changelog:header

We added "Action Mailbox Basics", "Action Text Overview" guides(#34812, #34878)
I think it makes to mention about it in the changelog file. (Similar to 7200ec92)
Note that entries retain original author since
I just moved content from readme files to the guides.

Generally followed the pattern for https://github.com/rails/rails/pull/32034

* Removes needless CI configs for 2.4
* Targets 2.5 in rubocop
* Updates existing CHANGELOG entries for fewer merge conflicts
* Removes Hash#slice extension as that's inlined on Ruby 2.5.
* Removes the need for send on define_method in MethodCallAssertions.

We only add the header when releasing to avoid some conflicts.

[ci skip]

Skipping over 2.4.0 to sidestep the `"symbol_from_string".to_sym.dup` bug.

References #32028

:tada::tada::tada:

[ci skip]

- Made all sentences to end properly
[ci skip]

- This change is made as the behavior for `:message` proc was changed in
  https://github.com/rails/rails/pull/24119.
- Also check
  https://github.com/rails/rails/pull/24431#issuecomment-206106790 for
  reference.

Adds changelog headers for beta3 release