Fix FirebaseApp sweepers

[modular-magician]

These sweepers have been reporting that there are no sweepable resources, but there are resources accumulating and they have hit the limit. This makes a few changes to get the sweeper working as intended:

(One of the API specs for reference https://firebase.google.com/docs/reference/firebase-management/rest/v1beta1/projects.iosApps)

• Fix the field used from the list response to apps
• Fix the remove request to use POST
• Fix the sweeper sweepable field to be displayName instead of name, because name is server-generated
• Fix the displayNames used for FirebaseApp tests to include sweepable prefixes

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
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Derived from GoogleCloudPlatform/magic-modules#7279