Do not support GCC 9 (#1978)

This will let us use C++20 features that GCC 9's experimental
C++20 support did not yet cover, such as "concepts".

This reverts some commits:
- 6bcd79b997
- d5ce5329ea
- 728d14879b

.github/scripts/install_deps.sh

@@ -20,7 +20,7 @@ case "${OS%%-*}" in
 				pkgs="$pkgs libz-mingw-w64-dev g++-mingw-w64-x86-64-win32"
 				TOOLSET=
 			;;
-			g++-9 | lcov)
+			g++-10 | lcov)
 				pkgs="$pkgs libpng-dev pkgconf $TOOLSET"
 				TOOLSET=
 			;;