vagrant: remove debian 9 "stretch" box

there are multiple issues with that box:
- debian 9 is out of support by debian, out of even lts support since 2022
- it has a OpenSSL 1.x that is also out of support and noone will care for it.
- has python 3.5

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

Vagrantfile

@@ -342,22 +342,6 @@ Vagrant.configure(2) do |config|
     b.vm.provision "run tests", :type => :shell, :privileged => false, :inline => run_tests("buster64", ".*none.*")
   end
 
-  config.vm.define "stretch64" do |b|
-    b.vm.box = "generic/debian9"
-    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 "install pyenv", :type => :shell, :privileged => false, :inline => install_pyenv("stretch64")
-    b.vm.provision "install pythons", :type => :shell, :privileged => false, :inline => install_pythons("stretch64")
-    b.vm.provision "build env", :type => :shell, :privileged => false, :inline => build_pyenv_venv("stretch64")
-    b.vm.provision "install borg", :type => :shell, :privileged => false, :inline => install_borg("llfuse")
-    b.vm.provision "install pyinstaller", :type => :shell, :privileged => false, :inline => install_pyinstaller()
-    b.vm.provision "build binary with pyinstaller", :type => :shell, :privileged => false, :inline => build_binary_with_pyinstaller("stretch64")
-    b.vm.provision "run tests", :type => :shell, :privileged => false, :inline => run_tests("stretch64", ".*(fuse3|none).*")
-  end
-
   config.vm.define "freebsd64" do |b|
     b.vm.box = "generic/freebsd14"
     b.vm.provider :virtualbox do |v|