Upgrade appservice-slack (2.0.2 -> 2.1.2)

Related to https://matrix.org/blog/2023/07/bridges-security-updates/

roles/custom/matrix-bridge-appservice-slack/defaults/main.yml

@@ -11,7 +11,7 @@ matrix_appservice_slack_docker_src_files_path: "{{ matrix_base_data_path }}/apps
 
 # matrix_appservice_slack_version used to contain the full Docker image tag (e.g. `release-X.X.X`).
 # It's a bare version number now. We try to somewhat retain compatibility below.
-matrix_appservice_slack_version: 2.0.2
+matrix_appservice_slack_version: 2.1.2
 matrix_appservice_slack_docker_image: "{{ matrix_container_global_registry_prefix }}matrixdotorg/matrix-appservice-slack:{{ matrix_appservice_slack_docker_image_tag }}"
 matrix_appservice_slack_docker_image_tag: "{{ 'latest' if matrix_appservice_slack_version == 'latest' else ('release-' + matrix_appservice_slack_version) }}"
 matrix_appservice_slack_docker_image_force_pull: "{{ matrix_appservice_slack_docker_image.endswith(':latest') }}"