Rename variable (matrix_hookshot_feeds_interval -> matrix_hookshot_feeds_pollIntervalSeconds)

This is more consistent with how we name variables. It's also less
confusing, especially given that we have `matrix_hookshot_feeds_pollTimeoutSeconds` as well.

roles/matrix-bridge-hookshot/tasks/validate_config.yml

@@ -58,6 +58,15 @@
   with_items:
     - "matrix_hookshot_provisioning_secret"
 
+- name: (Deprecation) Catch and report renamed Hookshot variables
+  ansible.builtin.fail:
+    msg: >-
+      Your configuration contains a variable, which now has a different name.
+      Please change your configuration to rename the variable (`{{ item.old }}` -> `{{ item.new }}`).
+  when: "item.old in vars"
+  with_items:
+    - {'old': 'matrix_hookshot_feeds_interval', 'new': 'matrix_hookshot_feeds_pollIntervalSeconds'}
+
 - name: (Deprecation) Catch and report old metrics usage
   ansible.builtin.fail:
     msg: >-