[doc] Increase minimum supported GCC version to 9

Since the legacy GCC docker image doesn't (readily) support the
required CMake version 3.18, the CI was updated to use GCC 9.

GCC 8 may continue to work, and one could possibly add CMake 3.18 to
the old Debian image to make a CI for it work again, but it's just not
worth the effort at this time: GCC 9 was released in 2019, so it's old
enough to be widely supported. Just as, or even more so than, GCC 8
was when this project started.

README.md

@@ -9,7 +9,7 @@ was made open source software in May 2005, released under a BSD-style license. 
 
 ## Modern Toolchain Port
 
-This fork is a **MingW-w64 GCC 8+** as well as **MSVC 19+** port of AVS.
+This fork is a **MingW-w64 GCC 9+** as well as **MSVC 19+** port of AVS.
 
 The goal was to create a **v2.81d**-compliant *vis_avs.dll* version that can be built
 with a modern toolchain, which was largely successful. One notable exception being that