Switch matrix-media-repo to Github Container Registry (supports multi-arch)

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3349

`docker.io/turt2live/matrix-media-repo:v1.3.4` is amd64-only.

`ghcr.io/t2bot/matrix-media-repo:v1.3.4` is a multi-arch image which
works on arm64.

roles/custom/matrix-media-repo/defaults/main.yml

@@ -17,7 +17,7 @@ matrix_media_repo_container_image_self_build_repo: "https://github.com/turt2live
 
 matrix_media_repo_docker_image_path: "turt2live/matrix-media-repo"
 matrix_media_repo_docker_image: "{{ matrix_media_repo_docker_image_name_prefix }}{{ matrix_media_repo_docker_image_path }}:{{ matrix_media_repo_docker_image_tag }}"
-matrix_media_repo_docker_image_name_prefix: "{{ 'localhost/' if matrix_media_repo_container_image_self_build else matrix_container_global_registry_prefix }}"
+matrix_media_repo_docker_image_name_prefix: "{{ 'localhost/' if matrix_media_repo_container_image_self_build else 'ghcr.io/' }}"
 # renovate: datasource=docker depName=turt2live/matrix-media-repo
 matrix_media_repo_docker_image_tag: "v1.3.4"
 matrix_media_repo_docker_image_force_pull: "{{ matrix_media_repo_docker_image.endswith(':latest') }}"