[Auth] Remove deprecated pre-iOS 13.5 GameCenter workaround and fix force-unwrap

[crleonard]

Closes #16135

Description

GameCenterAuthProvider.swift contained a WarningWorkaround protocol and
pre135Credential() method added to suppress deprecation warnings for
GKLocalPlayer.playerID and generateIdentityVerificationSignature. These
existed to support iOS < 13.5 before fetchItems was available, and were
explicitly marked for deletion:

// TODO: Delete this when minimum iOS version passes 13.5.

The SDK minimum is now iOS 15 (Package.swift), so this condition is satisfied.

Additionally, the async getCredential() overload force-unwrapped error! with
a TODO noting it should be replaced — swapped for a safe
AuthErrorUtils.error(code: .internalError) fallback.

Changes

• Removed WarningWorkaround private protocol
• Removed extension GameCenterAuthProvider: WarningWorkaround
• Removed #available(iOS 13.5, ...) runtime branch and the else fallback
• Removed pre135Credential() method (~30 lines of dead code)
• Fixed force-unwrap error! → safe fallback in async getCredential()

Testing

Existing SignInWithGameCenterTests pass with no changes required:

• testGameCenterAuthCredentialCoding ✓
• testRequestResponseEncoding ✓

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[paulb777]

Thanks for the PR! Based on the CI failures, it looks like we still need this code as long as we're supporting macOS 10.15

[ncooke3]

I'll add it to our major version backlog for consideration. If the macOS minimum version is raised then, we can revive this PR.