Bump diagnostic_namespace rust version

[mejrs]

Now that rust 1.79 is out, I'd like to avoid using diagnostic_namespace on 1.78. Because we don't test CI for 1.78 it seems prudent to not use it for that version.

[davidhewitt]

Makes sense 👍

I guess in general we only need to do this because of the ICE, rather than every stable release?

[mejrs]

I guess in general we only need to do this because of the ICE, rather than every stable release?

Unless new ices or other gotchas are discovered, yes :)