minifiers: Update deprecation handling

minifiers: Update deprecation handling

1. With minify.tdewolff.css and minify.tdewolff.svg, check
   for "decimals" instead of "decimal"

2. Add deprecation messages for:

  - minify.tdewolff.css.decimals
  - minify.tdewolff.svg.decimals
  - minify.tdewolff.html.keepconditionalcomments
  - config root minify (bool)
  - config root minifyoutput (bool)

3. Deprecate minify.tdewolff.css.keepcss2 in favor
   of minify.tdewolff.css.version

4. Refactor minify configuration tests

Closes #11893
Closes #13947
Closes #13948