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-client-element/tasks/prepare_themes.yml

@@ -8,6 +8,7 @@
   - name: Ensure Element themes repository is pulled
     ansible.builtin.git:
       repo: "{{ matrix_client_element_themes_repository_url }}"
+      version: "{{ matrix_client_element_themes_repository_version }}"
       dest: "{{ role_path }}/files/scratchpad/themes"
 
   - name: Find all Element theme files