maint: remove 0.21 from maintenance-table.md

Author: fanquake

_includes/posts/maintenance-table.md

@@ -1,4 +1,3 @@
-| 0.21 | 2021-01-15 | 2022-04-25  | 2022-10-01 |
 | 22.0 | 2021-09-13 | 2022-11-24  | 2023-04-01 |
 | 23.0 | 2022-04-25 | after v25.0 | TBA        |
 | 24.0 | 2022-11-24 | after v26.0 | TBA        |