Make reference pitch for \relative non-optional

Make reference pitch for \relative non-optional

This is intended as a potential migration strategy to issue 3229 where
leaving off the optional reference pitch leads to a different default.
Any program not continuing to work after this change and the related
conversion would be likely to cause trouble then.

Using \relative without reference pitch has been deprecated for a long
time, so this change brings LilyPond's codebase (and files converted
using convert-ly) into conformance with recommended practices.


Obviously includes the commit

Run scripts/auxiliar/update-with-convert-ly.sh