ipabackup: Bump minimum supported Ansible version to 2.15

ansible-freeipa roles do not work with Ansible 2.8 anymore, so the
minimum supported version is changed to 2.15, the oldest supported
Ansible version as of today.

roles/ipabackup/README.md

@@ -42,7 +42,7 @@ Requirements
 ------------
 
 **Controller**
-* Ansible version: 2.13+
+* Ansible version: 2.15+
 
 **Node**
 * Supported FreeIPA version (see above)

roles/ipabackup/meta/main.yml

@@ -6,7 +6,7 @@ galaxy_info:
   description: A role to backup and restore an IPA server
   company: Red Hat, Inc
   license: GPLv3
-  min_ansible_version: "2.13"
+  min_ansible_version: "2.15"
   platforms:
   - name: Fedora
     versions: