vagrant: remove ubuntu 20.04 "focal" box

there are multiple issues with that box:
- it has a OpenSSL 1.x that is out of upstream support.
- has python 3.8

also:
- conservative users who did not upgrade yet to 22.04 will likely not jump onto borg 1.4 soon, but rather use borg 1.2.x.

We'll try to keep this box alive for borg 1.2.x, but for borg 1.4 it is "game over".

Vagrantfile

@@ -270,18 +270,6 @@ Vagrant.configure(2) do |config|
     v.cpus = $cpus
   end
 
-  config.vm.define "focal64" do |b|
-    b.vm.box = "ubuntu/focal64"
-    b.vm.provider :virtualbox do |v|
-      v.memory = 1024 + $wmem
-    end
-    b.vm.provision "fs init", :type => :shell, :inline => fs_init("vagrant")
-    b.vm.provision "packages debianoid", :type => :shell, :inline => packages_debianoid("vagrant")
-    b.vm.provision "build env", :type => :shell, :privileged => false, :inline => build_sys_venv("focal64")
-    b.vm.provision "install borg", :type => :shell, :privileged => false, :inline => install_borg("llfuse")
-    b.vm.provision "run tests", :type => :shell, :privileged => false, :inline => run_tests("focal64", ".*none.*")
-  end
-
   config.vm.define "jammy64" do |b|
     b.vm.box = "ubuntu/jammy64"
     b.vm.provider :virtualbox do |v|