Mk/bsd.default-versions.mk: Change default version of Ruby

• Change default version of Ruby from 3.0 to 3.1.
• Bump PORTREVISION of lang/ruby3[01]. They need to be rebuilt as only default ruby package includes ${PREFIX}/bin/ruby and ${PREFIX}/man/man1/ruby.1.gz symlinks.
• Add entry about change of default Ruby version to UPDATING.

PR: 268586
Exp-run by: antoine
Approved by: meta (ruby, maintainer)