Mostly CI/CD work

But also, small changes in the API-s.

• Added parameter isUtf8 to the Ulid constructor Ulid(ReadOnlySpan<byte>, bool isUtf8) and to bool TryWrite(Span<byte> destination, bool asUtf8).
  Instead of guessing the meaning of the bytes from the length, let the caller state clearly what is their intent.

• Small improvement of ToString().