Fix UstFlagParser bug - #2265

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-dotnet@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Nullability of reference types in type of parameter 'value' doesn't match implemented member 'int IList.Add(object? value)' (possibly because of nullability attributes).

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

'JapaneseMonophoneG2p.LoadPack(byte[], Func<string, string>, Func<string, string>)' hides inherited member 'G2pPack.LoadPack(byte[], Func<string, string>, Func<string, string>)'. Use the new keyword if hiding was intended.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.