gitlab#326 - COMPILING.md: Fix confusion about Ubuntu versions

GNOME/librsvg#326
3 jobs from tree-leak in 9 minutes and 6 seconds (queued for 4 minutes and 34 seconds)
Status Job ID Name Coverage
  Test
passed #92725
fedora test

08:43

 
  Lint
manual #92727
allowed to fail manual
clippy
passed #92726
rustfmt

00:22