Switch Synapse to being pulled from ghcr.io, not docker.io

Related to:

- https://github.com/matrix-org/synapse/pull/15281
- https://github.com/matrix-org/synapse/pull/15282

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

@@ -69,11 +69,11 @@ matrix_synapse_container_image_customizations_templates_git_repository_keyscan_h
 #  RUN echo 'You do NOT need to include a FROM clause yourself.'
 matrix_synapse_container_image_customizations_dockerfile_body_custom: ''
 
-matrix_synapse_docker_image: "{{ matrix_synapse_docker_image_name_prefix }}matrixdotorg/synapse:{{ matrix_synapse_docker_image_tag }}"
+matrix_synapse_docker_image: "{{ matrix_synapse_docker_image_name_prefix }}matrix-org/synapse:{{ matrix_synapse_docker_image_tag }}"
 matrix_synapse_docker_image_name_prefix: "{{ 'localhost/' if matrix_synapse_container_image_self_build else matrix_synapse_docker_image_registry_prefix }}"
 matrix_synapse_docker_image_tag: "{{ matrix_synapse_version }}"
 matrix_synapse_docker_image_force_pull: "{{ matrix_synapse_docker_image.endswith(':latest') }}"
-matrix_synapse_docker_image_registry_prefix: "{{ matrix_container_global_registry_prefix }}"
+matrix_synapse_docker_image_registry_prefix: ghcr.io/
 
 # matrix_synapse_docker_image_customized is the name of the locally built Synapse image
 # which adds various customizations on top of the original (upstream) Synapse image.