Rename Synapse worker preset name (room-workers -> specialized-workers)

I believe `specialized-workers` is a better name than `room-workers`,
because when enabled, 4 different types of specialized workers are
created:

- Room workers
- Sync workers
- Client readers
- Federation readers

Only one of these is called room-workers.

In the future, more specialized workers may be added, making the
`room-workers` preset name an even poorer choice.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3100

roles/custom/matrix-synapse/defaults/main.yml

@@ -706,7 +706,7 @@ matrix_synapse_workers_presets:
     stream_writer_account_data_stream_workers_count: 1
     stream_writer_receipts_stream_workers_count: 1
     stream_writer_presence_stream_workers_count: 1
-  room-workers:
+  specialized-workers:
     room_workers_count: 1
     sync_workers_count: 1
     client_reader_workers_count: 1

roles/custom/matrix-synapse/tasks/init.yml

@@ -1,5 +1,16 @@
 ---
 
+# This validation task is here, not in validate_config.yml,
+# because init.yml runs before it.
+#
+# validate_config.yml aims to validate the configuration based on the work we do,
+# so we can't change the order.
+- name: Fail when using the old (renamed) room-workers preset
+  ansible.builtin.fail:
+    msg: >-
+      The `room-workers` preset has been renamed to `specialized-workers`. Update your `matrix_synapse_workers_preset` variable to use the new name.
+  when: matrix_synapse_workers_preset == 'room-workers'
+
 # Unless `matrix_synapse_workers_enabled_list` is explicitly defined,
 # we'll generate it dynamically.
 - ansible.builtin.include_tasks: "{{ role_path }}/tasks/synapse/workers/init.yml"