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

@@ -1,8 +1,8 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|