Fix git-latest ansible-lint errors

Reference: https://ansible-lint.readthedocs.io/en/latest/default_rules/#git-latest

Our variable naming is not necessarily consistent across roles.
I've tried to follow the naming conventions of each individual role.
All new variables are suffixed with `_version`, but the prefix may be
somewhat different.

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

@@ -6,10 +6,14 @@ matrix_appservice_slack_enabled: true
 
 matrix_appservice_slack_container_image_self_build: false
 matrix_appservice_slack_docker_repo: "https://github.com/matrix-org/matrix-appservice-slack.git"
+matrix_appservice_slack_docker_repo_version: "{{ 'master' if matrix_appservice_slack_version == 'latest' else matrix_appservice_slack_version }}"
 matrix_appservice_slack_docker_src_files_path: "{{ matrix_base_data_path }}/appservice-slack/docker-src"
 
-matrix_appservice_slack_version: release-1.11.0
-matrix_appservice_slack_docker_image: "{{ matrix_container_global_registry_prefix }}matrixdotorg/matrix-appservice-slack:{{ matrix_appservice_slack_version }}"
+# 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: 1.11.0
+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') }}"
 
 matrix_appservice_slack_base_path: "{{ matrix_base_data_path }}/appservice-slack"

roles/matrix-bridge-appservice-slack/tasks/setup_install.yml

@@ -46,6 +46,7 @@
 - name: Ensure matrix-appservice-slack repository is present when self-building
   ansible.builtin.git:
     repo: "{{ matrix_appservice_slack_docker_repo }}"
+    version: "{{ matrix_appservice_slack_docker_repo_version }}"
     dest: "{{ matrix_appservice_slack_docker_src_files_path }}"
     force: "yes"
   become: true