Vagrantfile: Remove workarounds for ancient Vagrant bugs.

Vagrant 2.2.6 is the earliest version with the Docker usability test
fix, and is also conveniently the version in the Ubuntu 20.04
repository.  Users with older versions will get a friendly error
message from ‘Vagrant.require_version’ (and can install an updated
Vagrant from upstream).

Signed-off-by: Anders Kaseorg <anders@zulip.com>

Vagrantfile

@@ -1,45 +1,8 @@
 # -*- mode: ruby -*-
 
-VAGRANTFILE_API_VERSION = "2"
-
-if Vagrant::VERSION == "1.8.7"
-  path = `command -v curl`
-  if path.include?("/opt/vagrant/embedded/bin/curl")
-    puts "In Vagrant 1.8.7, curl is broken. Please use the latest Vagrant."
-    puts "See https://github.com/mitchellh/vagrant/issues/7997 for details."
-    exit
-  end
-end
-
-# Workaround: Vagrant removed the atlas.hashicorp.com to
-# vagrantcloud.com redirect in February 2018. The value of
-# DEFAULT_SERVER_URL in Vagrant versions less than 1.9.3 is
-# atlas.hashicorp.com, which means that removal broke the fetching and
-# updating of boxes (since the old URL doesn't work).  See
-# https://github.com/hashicorp/vagrant/issues/9442
-if Vagrant::DEFAULT_SERVER_URL == "atlas.hashicorp.com"
-  Vagrant::DEFAULT_SERVER_URL.replace("https://vagrantcloud.com")
-end
-
-# Monkey patch https://github.com/hashicorp/vagrant/pull/10879 so we
-# can fall back to another provider if docker is not installed.
-begin
-  require Vagrant.source_root.join("plugins", "providers", "docker", "provider")
-rescue LoadError
-else
-  VagrantPlugins::DockerProvider::Provider.class_eval do
-    method(:usable?).owner == singleton_class or def self.usable?(raise_error = false)
-      VagrantPlugins::DockerProvider::Driver.new.execute("docker", "version")
-      true
-    rescue Vagrant::Errors::CommandUnavailable, VagrantPlugins::DockerProvider::Errors::ExecuteError
-      raise if raise_error
-      return false
-    end
-  end
-end
-
-Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
+Vagrant.require_version ">= 2.2.6"
 
+Vagrant.configure("2") do |config|
   # The Zulip development environment runs on 9991 on the guest.
   host_port = 9991
   http_proxy = https_proxy = no_proxy = nil