Improve compile guard in digest_spec.c

[herwinw]

This macro should be available from Ruby 4.0 and onwards. Make this more explicit.

[herwinw]

I guess this is a bit of a chicken and egg question, because now to run the digest specs with Ruby 4.0 compatibility, you need to define this macro.
This is mostly relevant to Truffleruby, so the correct people to judge this change are active in this repo ;)