corrects typo in bar-line.scm

issue 3908

Missing space in (define-bar-line ":|]" ":|]" #f " |") caused bad
visual output of VoltaBracket.
Changed to (define-bar-line ":|]" ":|]" #f " | ")
The space added will cause that the SpanBar x-extent will be calculated
correctly to determine the gap between two VoltaBrackets.
CG: Updated Minor Release Checklist

To include the changing of the \version number of the two

Welcome* files in the ly directory

Currently this is hard coded.