Clean up dirty tracking for integration fields

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Summary

Follow-up from !81813 (comment 912180070):

🤔 Unrelated to your MR, but it looks like nothing ever resets @old_data_fields in our code, even after commit.

• Changes to properties are tracked via @updated_properties
• Changes to data_fields are tracked via @old_data_fields
• When saving the integration record, we only reset @updated_properties via the after_commit hook at https://gitlab.com/gitlab-org/gitlab/blob/eb83f947442c090b3c66dec0c59602e6a6561eec/app/models/integration.rb#L90
• A consequence of this is that e.g. #{field}_changed? still returns true after saving for data fields, but false for properties.
• This doesn't seem to cause any bugs (AFAIK), so this would be just a clean-up.

Improvements

We could clear @old_data_fields too in reset_updated_properties, but should probably rename the method for clarity (reset_updated_fields?).

Maybe we could even use the same instance variable for both properties and data fields, since we shouldn't really use both at the same time anyway.