Add validation tasks for Etherpad migration (matrix_etherpad -> etherpad)

roles/custom/matrix_playbook_migration/defaults/main.yml

@@ -17,3 +17,6 @@ matrix_playbook_migration_matrix_ntfy_migration_validation_enabled: true
 
 # Controls if (`matrix_redis` -> `redis`) validation will run.
 matrix_playbook_migration_matrix_redis_migration_validation_enabled: true
+
+# Controls if (`matrix_etherpad` -> `etherpad`) validation will run.
+matrix_playbook_migration_matrix_etherpad_migration_validation_enabled: true

roles/custom/matrix_playbook_migration/tasks/validate_config.yml

@@ -141,3 +141,18 @@
           Please change your configuration (vars.yml) to rename all variables (`matrix_redis_` -> `redis_`).
           We found usage of the following variables: {{ matrix_playbook_migration_redis_migration_vars.keys() | join(', ') }}
       when: "matrix_playbook_migration_redis_migration_vars | length > 0"
+
+- when: matrix_playbook_migration_matrix_etherpad_migration_validation_enabled | bool
+  block:
+    - ansible.builtin.set_fact:
+        matrix_playbook_migration_etherpad_migration_vars: |-
+          {{ vars | dict2items | selectattr('key', 'match', 'matrix_etherpad_.*') | list | items2dict }}
+
+    - name: (Deprecation) Catch and report matrix_etherpad variables
+      ansible.builtin.fail:
+        msg: >-
+          The matrix-etherpad role that used to be part of this playbook has been replaced by https://gitlab.com/etke.cc/roles/etherpad.
+          The new role is compatible with the old one, but uses different names for its variables.
+          Please change your configuration (vars.yml) to rename all variables (`matrix_etherpad_` -> `etherpad_`).
+          We found usage of the following variables: {{ matrix_playbook_migration_etherpad_migration_vars.keys() | join(', ') }}
+      when: "matrix_playbook_migration_etherpad_migration_vars | length > 0"