Latest GTMSessionFetcher's new deprecation breaks Storage CI

[paulb777]

See https://github.com/firebase/firebase-ios-sdk/pull/8292/checks?check_run_id=2898960029 and

From GTMsessionFetcherService.h:
// DEPRECATED: Callers should use XCTestExpectation instead.
//
// Spin the run loop and discard events (or, if not on the main thread, just sleep the thread)
// until all running and delayed fetchers have completed.
//
// This is only for use in testing or in tools without a user interface.
//
// Synchronous fetches should never be done by shipping apps; they are
// sufficient reason for rejection from the app store.

Given that comment, it seems we might need a more significant refactor than most warnings require.

cc: @thomasvl

[schmidt-sebastian]

@morganchen12 - Can you take care of this?

[thomasvl]

@mwyman fyi also

I believe the api docs have always said it should only be used in tests (or tools without UI).