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=
 			;;

.github/workflows/testing.yml

@@ -40,9 +40,9 @@ jobs:
         exclude: # Don't use `g++` on macOS; it's just an alias to `clang++`.
           - { os: macos-15-intel, cxx: g++ }
           - { os: macos-26, cxx: g++ }
-        include: # Use `g++-9`, the earliest GCC version we support, on the earliest Ubuntu runner.
-          - { os: ubuntu-22.04, cxx: g++-9, buildsys: make }
-          - { os: ubuntu-22.04, cxx: g++-9, buildsys: cmake }
+        include: # Use `g++-10`, the earliest GCC version we support, on the earliest Ubuntu runner.
+          - { os: ubuntu-22.04, cxx: g++-10, buildsys: make }
+          - { os: ubuntu-22.04, cxx: g++-10, buildsys: cmake }
       fail-fast: false
     runs-on: ${{ matrix.os }}
     steps: