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

app/models/account.rb

@@ -54,13 +54,13 @@ class Account < ApplicationRecord
   validates :username, presence: true
 
   # Remote user validations
-  validates :username, uniqueness: { scope: :domain, case_sensitive: true }, if: -> { !local? && username_changed? }
+  validates :username, uniqueness: { scope: :domain, case_sensitive: true }, if: -> { !local? && will_save_change_to_username? }
 
   # Local user validations
-  validates :username, format: { with: /\A[a-z0-9_]+\z/i }, uniqueness: { scope: :domain, case_sensitive: false }, length: { maximum: 30 }, if: -> { local? && username_changed? }
-  validates_with UnreservedUsernameValidator, if: -> { local? && username_changed? }
-  validates :display_name, length: { maximum: 30 }, if: -> { local? && display_name_changed? }
-  validates :note, length: { maximum: 160 }, if: -> { local? && note_changed? }
+  validates :username, format: { with: /\A[a-z0-9_]+\z/i }, uniqueness: { scope: :domain, case_sensitive: false }, length: { maximum: 30 }, if: -> { local? && will_save_change_to_username? }
+  validates_with UnreservedUsernameValidator, if: -> { local? && will_save_change_to_username? }
+  validates :display_name, length: { maximum: 30 }, if: -> { local? && will_save_change_to_display_name? }
+  validates :note, length: { maximum: 160 }, if: -> { local? && will_save_change_to_note? }
 
   # Timelines
   has_many :stream_entries, inverse_of: :account, dependent: :destroy