Update FreeBSD build to use gcc-10

Makefile

 		cd /vagrant && \
 		python3.7 tools/ci.py \
 			-B download \
-			-T tools/freebsd-gcc-9.jsonc \
+			-T tools/freebsd-gcc-10.jsonc \
 		'
 	vagrant scp freebsd11:/vagrant/_build/dds _build/dds-freebsd-x64
 	vagrant halt
 	mkdir -p _site/
 	cp site/index.html _site/
 	cp -r _build/docs _site/
-	echo "Site generated at _site/"
+	echo "Site generated at _site/"

Vagrantfile

     freebsd11.vm.box = 'generic/freebsd11'
     freebsd11.vm.provision 'shell', inline: <<-SHELL
       set -eu
-      for package in python37 py37-pip ccache gcc9 gcc8 git; do
+      for package in python37 py37-pip ccache gcc10 gcc9 gcc8 git; do
         echo "Installing $package"
         pkg install -qy $package
       done

tools/freebsd-gcc-9.jsonc → tools/freebsd-gcc-10.jsonc

 {
     "$schema": "../res/toolchain-schema.json",
     "compiler_id": "gnu",
-    "c_compiler": "gcc9",
-    "cxx_compiler": "g++9",
+    "c_compiler": "gcc10",
+    "cxx_compiler": "g++10",
+    "cxx_version": "c++20",
     "warning_flags": [
         "-Werror",
     ],
-    "cxx_flags": [
-        "-fconcepts",
-        "-std=c++2a",
-    ],
     "link_flags": [
         "-static-libgcc",
         "-static-libstdc++"