RuntimeHintsWriter should comply with reachability-metadata-schema-v1.2.0.json

[snicoll]

GraalVM 25 is now promoting v1.2.0 but we're still on an old version.

[snicoll]

Looking at the proposal, the @Nullable for serializable is really annoying, it should be a primitive as the default value is obvious. I'll revisit this in a polish commit.