Fix class_attribute shared with parent class

What does this MR do and why?

https://api.rubyonrails.org/classes/Class.html#method-i-class_attribute

This matches normal Ruby method inheritance: think of writing an attribute on a subclass as overriding the reader method. However, you need to be aware when using class_attribute with mutable structures as Array or Hash. In such cases, you don't want to do changes in place. Instead use setters:

Base.setting = []
Base.setting                # => []
Subclass.setting            # => []

# Appending in child changes both parent and child because it is the same object:
Subclass.setting << :foo
Base.setting               # => [:foo]
Subclass.setting           # => [:foo]

# Use setters to not propagate changes:
Base.setting = []
Subclass.setting += [:foo]
Base.setting               # => []
Subclass.setting           # => [:foo]

Screenshots or screen recordings

Before :

[1] pry(main)> WebHook.attributes_exempt_from_serializable_hash
=> [:runners_registration_token,
 "runners_registration_token_encrypted",
 :health_check_access_token,
 :static_objects_external_storage_auth_token,
 "static_objects_external_storage_auth_token_encrypted",
 :incoming_email_token,
 :feed_token,
 :static_object_token,
 "static_object_token_encrypted",
 :runners_token,
 "runners_token_encrypted",
 :token,
 "token_expires_at",
 "token_encrypted",
 :token,
 "token_encrypted",
 :token,
 "token_encrypted",
 :runners_token,
 "runners_token_encrypted",
 :saml_discovery_token]
[2] pry(main)> User.attributes_exempt_from_serializable_hash
=> [:runners_registration_token,
 "runners_registration_token_encrypted",
 :health_check_access_token,
 :static_objects_external_storage_auth_token,
 "static_objects_external_storage_auth_token_encrypted",
 :incoming_email_token,
 :feed_token,
 :static_object_token,
 "static_object_token_encrypted",
 :runners_token,
 "runners_token_encrypted",
 :token,
 "token_expires_at",
 "token_encrypted",
 :token,
 "token_encrypted",
 :token,
 "token_encrypted",
 :runners_token,
 "runners_token_encrypted",
 :saml_discovery_token]
[3] pry(main)> Project.attributes_exempt_from_serializable_hash
=> [:runners_registration_token,
 "runners_registration_token_encrypted",
 :health_check_access_token,
 :static_objects_external_storage_auth_token,
 "static_objects_external_storage_auth_token_encrypted",
 :incoming_email_token,
 :feed_token,
 :static_object_token,
 "static_object_token_encrypted",
 :runners_token,
 "runners_token_encrypted",
 :token,
 "token_expires_at",
 "token_encrypted",
 :token,
 "token_encrypted",
 :token,
 "token_encrypted",
 :runners_token,
 "runners_token_encrypted",
 :saml_discovery_token]

After:

[1] pry(main)> WebHook.attributes_exempt_from_serializable_hash
=> []
[2] pry(main)> User.attributes_exempt_from_serializable_hash
=> [:incoming_email_token, :feed_token, :static_object_token, "static_object_token_encrypted"]
[3] pry(main)> Project.attributes_exempt_from_serializable_hash
=> [:runners_token, "runners_token_encrypted"]

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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