Remove banned ToUpper and ToLower covered by CA1304

CodeAnalysis/BannedSymbols.txt

@@ -8,8 +8,4 @@ M:System.Threading.Tasks.Task.Wait();Don't use Task.Wait. Use Task.WaitSafely()
 M:System.Guid.#ctor;Probably meaning to use Guid.NewGuid() instead. If actually wanting empty, use Guid.Empty.
 P:System.Threading.Tasks.Task`1.Result;Don't use Task.Result. Use Task.GetResultSafely() to ensure we avoid deadlocks.
 M:System.Threading.ManualResetEventSlim.Wait();Specify a timeout to avoid waiting forever.
-M:System.Char.ToLower(System.Char);char.ToLower() changes behaviour depending on CultureInfo.CurrentCulture. Use char.ToLowerInvariant() instead. If wanting culture-sensitive behaviour, explicitly provide CultureInfo.CurrentCulture.
-M:System.Char.ToUpper(System.Char);char.ToUpper() changes behaviour depending on CultureInfo.CurrentCulture. Use char.ToUpperInvariant() instead. If wanting culture-sensitive behaviour, explicitly provide CultureInfo.CurrentCulture.
-M:System.String.ToLower();string.ToLower() changes behaviour depending on CultureInfo.CurrentCulture. Use string.ToLowerInvariant() instead. If wanting culture-sensitive behaviour, explicitly provide CultureInfo.CurrentCulture or use LocalisableString.
-M:System.String.ToUpper();string.ToUpper() changes behaviour depending on CultureInfo.CurrentCulture. Use string.ToUpperInvariant() instead. If wanting culture-sensitive behaviour, explicitly provide CultureInfo.CurrentCulture or use LocalisableString.
 M:System.Reflection.Assembly.GetEntryAssembly();Use osu.Framework.RuntimeInfo.EntryAssembly instead