Remove deprecated usage of `attribute_changed?`

What does this MR do?

Changes the deprecated usage of attribute_changed? and related *_was / *_changed? methods in after_save blocks

In Rails 5.2, the behavior of these methods would change when inside after_save.

The new method names for the old behavior are: saved_change_to_attribute?, *_before_last_save and saved_change_to_*?

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #9932 (closed)