Polish spring-javadoc.css.

Changes made:

  • Used shorthand properties where suitable
  • Removed redundant unit -px
  • Inserted white-spaces after the colon for better readability

Comment From: wilkinsona

Thanks for the PR. Reviewing it made me realise that the file you've polished is no longer used so I've deleted it in https://github.com/spring-projects/spring-boot/issues/42187.