CI: move Vagrant tests to GitHub Actions (larger runners)

We have been using Cirrus CI for running vagrant workloads
as the standard runners of GHA lacks nested virtualization,
but it looks like GHA with the "larger" runners support nested
virtualization.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Vagrantfile

@@ -325,29 +325,4 @@ EOF
     SHELL
   end
 
-  # Rootless Podman is used for testing CRI-in-UserNS
-  # (We could use rootless nerdctl, but we are using Podman here because it is available in dnf)
-  config.vm.provision "install-rootless-podman", type: "shell", run: "never" do |sh|
-    sh.upload_path = "/tmp/vagrant-install-rootless-podman"
-    sh.inline = <<~SHELL
-        #!/usr/bin/env bash
-        set -eux -o pipefail
-        # Delegate cgroup v2 controllers to rootless
-        mkdir -p /etc/systemd/system/user@.service.d
-        cat > /etc/systemd/system/user@.service.d/delegate.conf << EOF
-[Service]
-Delegate=yes
-EOF
-        systemctl daemon-reload
-        # Install Podman
-        dnf install -y podman
-        # Configure Podman to resolve `golang` to `docker.io/library/golang`
-        mkdir -p /etc/containers
-        cat > /etc/containers/registries.conf <<EOF
-[registries.search]
-registries = ['docker.io']
-EOF
-    SHELL
-  end
-
 end