CSS content update: learn to support older browsers (mdn#31473)

* supporting older browsers page

* remove more legacy cruft

* typo

* changes the entire see also

* Apply suggestions from code review

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

* Apply suggestions from code review

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

* Update files/en-us/learn/css/css_layout/supporting_older_browsers/index.md

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* rewrote selector section

* Apply suggestions from code review

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

* fix typo

---------

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>