Remove end of life ubuntu versions in CI (#10107)

* tests: replace ubuntu16 with ubuntu20

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

* tests: replace ubuntu18 with ubuntu20

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

* docs: update docs to remove support for ubuntu 16 and 18

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

* molecule: upgrade ubuntu versions

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

* vagrant: upgrade ubuntu versions

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

* tests: cleanup ubuntu{16,18}

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

* tests: increase ubuntu22 ram to allow molecule creation

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

---------

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

test-infra/image-builder/roles/kubevirt-images/defaults/main.yml

@@ -6,20 +6,6 @@ docker_host: quay.io
 registry: quay.io/kubespray
 
 images:
-  ubuntu-1604:
-    filename: xenial-server-cloudimg-amd64-disk1.img
-    url: https://storage.googleapis.com/kubespray-images/ubuntu/xenial-server-cloudimg-amd64-disk1.img
-    checksum: sha256:c0d099383cd064390b568e20d1c39a9c68ba864764404b70f754a7b1b2f808f7
-    converted: false
-    tag: "latest"
-
-  ubuntu-1804:
-    filename: bionic-server-cloudimg-amd64.img
-    url: https://cloud-images.ubuntu.com/bionic/current/bionic-server-cloudimg-amd64.img
-    checksum: sha256:c3d0e03f4245ffaabd7647e6dabf346b944a62b9934d0a89f3a04b4236386af2
-    converted: false
-    tag: "latest"
-
   ubuntu-2004:
     filename: focal-server-cloudimg-amd64.img
     url: https://cloud-images.ubuntu.com/focal/current/focal-server-cloudimg-amd64-disk-kvm.img