feat: Add BIND9 9.20 molecule scenario and support documentation

- Added molecule/bind9-20 scenario for testing BIND9 9.20+ compatibility
  - molecule.yml: Ubuntu 24.04 platform configuration
  - converge.yml: Complete 9.20 configuration with TLS, DNSTAP, and modern features
  - verify.yml: Comprehensive test cases for 9.20 features
  - collections.yml: Required Ansible collections
  - prepare.yml: Pre-test environment setup
  - README.md: Scenario documentation with breaking changes reference

- Added docs/BIND9_9.20_SUPPORT.md implementation guide
  - Architecture overview for multi-version support
  - Runtime version detection strategy
  - Configuration changes and examples
  - Migration path for upgrading users
  - Feature highlights for BIND9 9.20

- Updated meta/argument_specs.yml
  - Added multi-version support documentation
  - Documented bind9_version variable (read-only, auto-detected)
  - Clarified supported BIND9 versions (9.18.x LTS and 9.20+)

These changes establish the feature/bind9-20-support branch as the development
path for BIND9 9.20+ support, separate from the main branch's 9.18.x focus.

Closes #9: Create feature/bind9-20-support branch with 9.20 templates

meta/argument_specs.yml

@@ -2,6 +2,10 @@
 argument_specs:
   main:
     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.
     options:
       bind9_config:
         type: list
@@ -53,3 +57,10 @@ 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.