Set the ansible-core and Python versions of the workflows in one place #570

The ansible-core and Python versions are pinned in each workflow on its own (test_roles_pr.yml, the test_role_*.yml files, test_plugins.yml, test_cert_renewal.yml). Moving to a new release means editing all of them, and a file that is missed keeps the old version without anyone noticing, see #441.

The tested versions are also behind. ansible-core 2.18 reached end of life in May 2026 and 2.19 does in November 2026, but CI still tests both, while 2.21 and 2.22 are not tested. See Release and maintenance.

Define the versions once, use them in all workflows, move CI to the supported ansible-core releases and raise requires_ansible in meta/runtime.yml to the oldest tested one.