MAINT(readme): Remove mention of Windows 7

Win7 is no longer supported and therefore this statement was simply wrong and caused confusion - cmp. #6473

README.md

@@ -18,11 +18,6 @@ There are two modules in Mumble; the client (mumble) and the server (murmur).
 The client works on Windows, Linux, FreeBSD, OpenBSD, and macOS,
 while the server should work on anything Qt can be installed on.
 
-Please note that with "Windows" we mean 7 and newer.
-Vista may be supported, but we can't guarantee it.
-If you don't want to encounter potential issues, you may download Mumble 1.3.x,
-the last version to provide support for XP.
-
 The documentation of the project can be found on the [wiki](https://wiki.mumble.info/wiki/Main_Page). The
 [FAQ](https://wiki.mumble.info/wiki/FAQ/English) can also be found there.