Use styling directives as recommended in XEP-0393

Remove style *underline* and add style *strikethrough*.

Fixes #7613.
3 jobs for style in 3 minutes and 18 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
passed run-appdata #19882

00:00:05

passed run-tests #19881

00:03:02

 
  Build
passed run-build #19883

00:00:10