docs(requirements): Document new rolling support policy for browsers

Closes #5932

Implements the browser support policy as [discussed on the Elgg community][1].

[1]: https://community.elgg.org/discussion/view/1862271/browser-support-for-110

DO NOT MERGE until the core team has voted:

In favor of policy as worded in this PR:
 - @ewinslow
 - @beck24
 - @juho-jaakkola
 - @mrclay 

Opposed to policy as stated in this PR:
 - no one (yet)