fix: Update import path for get_placard_version_string

Fixed import in DiffStix.placard() to use the correct module path
after removing backward compatibility layer.

**Changes:**
- Changed import from changelog_helper to formatters.html_output
- get_placard_version_string is defined in formatters/html_output.py

**Test Results:**
- 132/133 tests passing (99.2%)
- Only failure is test_get_new_changelog_md_file_write_error (known permission issue)

Author: claude

mitreattack/diffStix/core/diff_stix.py

@@ -1050,7 +1050,7 @@ def placard(self, stix_object: dict, section: str, domain: str) -> str:
             Final return string to be displayed in the Changelog.
         """
         # Import here to avoid circular dependency
-        from mitreattack.diffStix.changelog_helper import get_placard_version_string
+        from mitreattack.diffStix.formatters.html_output import get_placard_version_string
 
         datastore_version = "old" if section == "deletions" else "new"
         placard_string = ""