docs: clarify BIND9 9.20 branch strategy

meta/argument_specs.yml

@@ -4,8 +4,7 @@ argument_specs:
     short_description: The main entry point for the bind9 role.
     description:
       - Configures BIND9 DNS server on Debian-based systems.
-      - "Supported BIND9 versions: 9.18.x (LTS), 9.20+ (feature releases)"
-      - Version detection is automatic at runtime.
+      - "Supported BIND9 versions are branch-specific: main targets 9.18.x (LTS), and the 9.20 branch targets 9.20+ feature releases."
     options:
       bind9_config:
         type: list
@@ -57,10 +56,3 @@ argument_specs:
       bind9_backup_dir:
         type: str
         description: Directory for backups.
-      bind9_version:
-        type: str
-        description:
-          - BIND9 version detected at runtime (read-only, set automatically).
-          - "Format: X.Y.Z (e.g., 9.18.44, 9.20.18)"
-          - Used by templates to apply version-specific configurations.
-          - Users should not set this variable directly.