[Infra] Disable Auth deprecations warnings in SPM CI

[ncooke3]

I enabled this for CocoaPods a while back and it works pretty well. Incrementally extending this to FirebaseAuth x SPM.

Example of usage:

firebase-ios-sdk/FirebaseAuth/Sources/Swift/Auth/Auth.swift

Lines 282 to 290 in ae25542

	#if !FIREBASE_CI
	@available(
	*,
	deprecated,
	message: "`fetchSignInMethods` is deprecated and will be removed in a future release. This method returns an empty list when Email Enumeration Protection is enabled."
	)
	#endif // !FIREBASE_CI
	@objc open func fetchSignInMethods(forEmail email: String,
	completion: (([String]?, Error?) -> Void)? = nil) {

#no-changelog

[andrewheard]

Nice, LGTM! Thanks for including the example in the PR description.