Deprecate `#clear_changed_attributes`

[no-reply]

This method can be replaced by ActiveModel::Dirty#changes_applied, which is more resiliant to changing ActiveModel and Rails behavior.

However, since we call it in a callback, and downstream users may be relying on that fact to trigger calls to their overrides, we can't avoid calling it locally until a 14.0.0 release.

[no-reply]

As noted, this method is hit by a callback. We have a few options:

• Merge this and emit deprecation notices on every save until the user upgrades to 14.0.0 (this is the only breaking change in 14.0.0).
• Introduce some metaprogramming to call a different method on callback, which would then emit a deprecation warning and route to the deprecated method only if it has been overridden.
• Do nothing; never deprecate, release 14.0.0 with the release notes

[cjcolvar]

Option 1 seems like it might overflow logs in CI. Option 2 seems better if it isn't too much work. For option 3, do we know if anyone has actually overridden this method? If not, then maybe the release notes are fine enough?

[coveralls]

Coverage decreased (-0.1%) to 89.482% when pulling 7edc40e3882804dbcf1179e2ee9bd6fb77e90620 on deprecate into 2b75d5ec94afc55fe4dafe01e55a27a11a466d79 on 13.x-stable.

[no-reply]

i think the proposed approach works, and may avoid the need to bump the major version #1405