roles: strict truthy values

According to Ansible we can use yes, true, True, "or any quoted st-
ring" for a boolean true, but ansible-lint wants us to use either
true or false.

See: https://chronicler.tech/red-hat-ansible-yes-no-and/

roles/mariadb/tasks/main.yml

@@ -11,7 +11,7 @@
 
 - name: Update apt cache
   ansible.builtin.apt:
-    update_cache: yes
+    update_cache: true
   when:
     add_mariadb_apt_key is changed or
     add_mariadb_apt_repository is changed