docs: update SECURITY.md

v8 has been released.

Author: abetomo

SECURITY.md

@@ -6,9 +6,9 @@ Old versions receive security updates for six months.
 
 | Version | Supported                                  |
 | ------- | ------------------------------------------ |
-| 7.x     | :white_check_mark:                         |
-| 6.x     | :white_check_mark: support ends 2021-06-30 |
-| < 6     | :x:                                        |
+| 8.x     | :white_check_mark:                         |
+| 7.x     | :white_check_mark: support ends 2022-01-31 |
+| < 7     | :x:                                        |
 
 Pull Requests for security issues will be considered for older versions back to 2.x.