Update deprecation status for version 6.x.x

Author: swmal

SECURITY.md

@@ -10,7 +10,7 @@ EPPlus 6, 7 and 8 are automatically scanned for vulnerabilities and static code
 | ------- | ------------------ | ------------------ |----|
 | 8.x.x   | :white_check_mark: |                    ||
 | 7.x.x   | :white_check_mark: |                    ||
-| 6.x.x   | :white_check_mark: |                    |2025-12-31|
+| 6.x.x   | :x:                |Deprecated/unsupported versions              |2025-12-31|
 | 5.x.x   | :x:                |Deprecated/unsupported versions              |2024-12-31|
 | < 4.3   | :x:                |Deprecated/unsupported versions|2020-12-31|