chore(ios): handle newly deprecated SDK constants

firebase-ios-sdk 8.15.0 deprecates this with the explicit message that
the values themselves will stay the same, and in their integration tests,
they simply in-lined the constants, so follow same approach

Author: mikehardy

packages/functions/ios/RNFBFunctions/RNFBFunctionsModule.m

@@ -62,8 +62,8 @@ @implementation RNFBFunctionsModule
                     NSObject *details = [NSNull null];
                     NSString *message = error.localizedDescription;
                     NSMutableDictionary *userInfo = [NSMutableDictionary dictionary];
-                    if (error.domain == FIRFunctionsErrorDomain) {
-                      details = error.userInfo[FIRFunctionsErrorDetailsKey];
+                    if ([error.domain isEqual:@"com.firebase.functions"]) {
+                      details = error.userInfo[@"details"];
                       if (details == nil) {
                         details = [NSNull null];
                       }