vagrant: update vagrant image to buster from jessie

I honestly have no idea if this works since I don't use vagrant, but
looking around online suggests it should work fine.

Differential Revision: https://phab.mercurial-scm.org/D7309

contrib/vagrant/Vagrantfile

@@ -1,13 +1,13 b''
 # -*- mode: ruby -*-
 
 Vagrant.configure('2') do |config|
-  # Debian 8.1 x86_64 without configuration management software
-  config.vm.box = "debian/jessie64"
+  # Debian 10.1 x86_64 without configuration management software
+  config.vm.box = "debian/buster64"
   config.vm.hostname = "tests"
 
   config.vm.define "tests" do |conf|
     conf.vm.provision :file, source: "run-tests.sh", destination:"run-tests.sh"
     conf.vm.provision :shell, path: "provision.sh"
     conf.vm.synced_folder "../..", "/hgshared"
   end
 end