print/lilypond-devel: update to 2.21.81

Several bugfixes and some improvements:

• Copy Balloon stencil before modifying
• musicxml: Ensure deterministic order of attributes
• musicxml: Remember order of voices
• Set up fallback fonts for Emmentaler

Changelogs:
https://lilypond.org/doc/v2.21/Documentation/changes/index.html
https://git.savannah.gnu.org/gitweb/?p=lilypond.git

PR: 251502
Submitted by: Martin Neubauer <m.ne@gmx.net> (maintainer)
Approved by: arrowd, tcberner (mentors, implicit)