Default I18n.enforce_available_locales to true

We will default this option to true from now on to ensure users properly
handle their list of available locales whenever necessary. This option
was added as a security measure and thus Rails will follow it defaulting
to secure option.

Also improve the handling of I18n config options in its railtie, taking
the new enforce_available_locales option into account, by setting it as
the last one in the process. This ensures no other configuration will
trigger a deprecation warning due to that setting.
Disable available locales checks to avoid warnings running the tests
Add changelog and upgrading notice related to I18n enforce_available_locales handling
Require I18n >= 0.6.9

The option enforce_available_locales is only available on latest
versions, so require the last available one which has the option +
other related fixes and should not have backward compatibility issues.
Merge pull request #13341 from carlosantoniodasilva/ca-i18n

Default I18n.enforce_available_locales to true

We will default this option to true from now on to ensure users properly handle their list of available locales whenever necessary. This option was added as a security measure and thus Rails will follow it defaulting to secure option.

Also improve the handling of I18n config options in its railtie, taking the new enforce_available_locales option into account, by setting it as the last one in the process. This ensures no other configuration will trigger a deprecation warning due to that setting.
Some assorted fixes for the 4.1 release notes:

* Added release notes for secrets.yml and mentioned it in the highlights
* Added release notes for Mailer previews and mentioned it in the highlights
* Added release notes for Module#concerning
* Removed mention for AV extraction from the highlights
* Rearranged the major features to put highlighted features first
* Various improvements and typo fixes

[ci skip]
Merge pull request #13346 from chancancode/release_notes_etc

Release notes fixes [ci skip]
sqlite >= 3.8.0 supports partial indexes