Adjust the minimum sdk version in the synthetic pkg for new Dart requirements (#122380)

Manually landing to fix the Engine rolls. The change will not affect Google Testing.

packages/flutter_tools/lib/src/commands/update_packages.dart

@@ -1440,7 +1440,7 @@ String generateFakePubspec(
   final bool verbose = doUpgrade;
   result.writeln('name: flutter_update_packages');
   result.writeln('environment:');
-  result.writeln("  sdk: '>=2.10.0 <4.0.0'");
+  result.writeln("  sdk: '>=2.12.0 <4.0.0'");
   result.writeln('dependencies:');
   overrides.writeln('dependency_overrides:');
   if (kManuallyPinnedDependencies.isNotEmpty) {