Allow form builder to open forms using deprecated metadata, but then force those metadata to be disabled

[jnm]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a description of your work suitable for publishing on our forum
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes: Document that simserial and subscriberid are deprecated docs#286

Description

The simserial and subscriberid metadata cannot be read from contemporary devices (#2842) and were removed entirely from the form builder in #3836, but that caused an error when trying to load old forms that had already enabled these metadata. This change allows those forms to be loaded without error, but the obsolete metadata are then disabled as soon as the form is saved.

Related issues

Part of #2842
Related to #3836