Update browser support targets following UI team discussion

Small but important changes:

- We should always be testing in Windows, even if MS Edge is now available on macOS as well.
- We should make a better effort to support older Safari releases, as Safari is tied to OS updates, and device management isn’t always keeping up with releases

docs/contributing/developing.rst

@@ -184,10 +184,10 @@ Mobile Safari  iOS Phone      Last 2
 Mobile Safari  iOS Tablet     Last 2
 Chrome         Android        Last 2
 Chrome         Desktop        Last 2
-MS Edge        Desktop        Last 2
+MS Edge        Windows        Last 2
 Firefox        Desktop        Latest
 Firefox ESR    Desktop        Latest
-Safari         macOS          Last 2
+Safari         macOS          Last 3
 =============  =============  =============
 
 We aim for Wagtail to work in those environments. Our development standards ensure that the site is usable on other browsers **and will work on future browsers**.