repo_proxysql: assert for missing builds and small cleanups #514

Leftovers found while fixing #506:

• ProxySQL 2.x has no EL 10 build on repo.proxysql.com; the role renders a URL that returns 404. An assert with a clear message would help.
• The get_url task leaves /tmp/ansible.RPM-GPG-KEY-proxysql on the Ansible controller.
• A task name has the typo RPM-PGP-KEY.
• autorefresh=1 in the repo file is a zypper option that dnf ignores.