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 22 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #18592
run-appdata

00:00:04

passed #18591
run-tests

00:03:07

 
  Build
passed #18593
run-build

00:00:10