maintenance: use .x to make it clearer the "branch" is maintained

In particular, the most-recent point release on the branch.

Author: fanquake

_includes/posts/maintenance-table.md

@@ -1,4 +1,4 @@
-| 25.0 | 2023-05-18 | 2024-04-16  | TBA        |
-| 26.0 | 2023-12-06 | after v28.0 | TBA        |
-| 27.0 | 2024-04-16 | after v29.0 | TBA        |
-| 28.0 | TBA*       | TBA         | TBA        |
+| 25.x | 2023-05-18 | 2024-04-16  | TBA        |
+| 26.x | 2023-12-06 | after v28.0 | TBA        |
+| 27.x | 2024-04-16 | after v29.0 | TBA        |
+| 28.x | TBA*       | TBA         | TBA        |