Accept incomplete version statements within stable releases

Commit 0ecab848 inadvertently changed behavior by rejecting incomplete version statements like \version "1" or \version "2.22". On the other hand, convert-ly has always rejected such versions. This brings consistency by accepting partial versions iff they have two digits, and the second is odd. This is because it can make sense to have \version "2.22", as syntax is supposed not to change within a stable release, whereas \version "2.23" is not useful to convert-ly because the syntax does change within an unstable release.