Fix Account model deprecation warnings (#3689)

```
DEPRECATION WARNING: The behavior of `attribute_changed?` inside of after callbacks will be changing in the next version of Rails. The new return value will reflect the behavior of calling the method after `save` returned (e.g. the opposite of what it returns now). To maintain the current behavior, use `saved_change_to_attribute?` instead. (called from block in <class:Account> at /Users/rene/Workspace/personal/ruby/mastodon/app/models/account.rb:60)
DEPRECATION WARNING: The behavior of `attribute_changed?` inside of after callbacks will be changing in the next version of Rails. The new return value will reflect the behavior of calling the method after `save` returned (e.g. the opposite of what it returns now). To maintain the current behavior, use `saved_change_to_attribute?` instead. (called from block in <class:Account> at /Users/rene/Workspace/personal/ruby/mastodon/app/models/account.rb:60)
DEPRECATION WARNING: The behavior of `attribute_changed?` inside of after callbacks will be changing in the next version of Rails. The new return value will reflect the behavior of calling the method after `save` returned (e.g. the opposite of what it returns now). To maintain the current behavior, use `saved_change_to_attribute?` instead. (called from block in <class:Account> at /Users/rene/Workspace/personal/ruby/mastodon/app/models/account.rb:60)
DEPRECATION WARNING: The behavior of `attribute_changed?` inside of after callbacks will be changing in the next version of Rails. The new return value will reflect the behavior of calling the method after `save` returned (e.g. the opposite of what it returns now). To maintain the current behavior, use `saved_change_to_attribute?` instead. (called from block in <class:Account> at /Users/rene/Workspace/personal/ruby/mastodon/app/models/account.rb:61)
DEPRECATION WARNING: The behavior of `attribute_changed?` inside of after callbacks will be changing in the next version of Rails. The new return value will reflect the behavior of calling the method after `save` returned (e.g. the opposite of what it returns now). To maintain the current behavior, use `saved_change_to_attribute?` instead. (called from block in <class:Account> at /Users/rene/Workspace/personal/ruby/mastodon/app/models/account.rb:62)
DEPRECATION WARNING: The behavior of `attribute_changed?` inside of after callbacks will be changing in the next version of Rails. The new return value will reflect the behavior of calling the method after `save` returned (e.g. the opposite of what it returns now). To maintain the current behavior, use `saved_change_to_attribute?` instead. (called from block in <class:Account> at /Users/rene/Workspace/personal/ruby/mastodon/app/models/account.rb:63)
```

Here's PR describing changes to Dirty API https://github.com/rails/rails/pull/25337