Add version 12 upgrade recommendations

Updates doc/policy/maintenance.md, adds upgrade path from 11 -> 12

doc/policy/maintenance.md

@@ -86,6 +86,7 @@ Please see the table below for some examples:
 | 9.4.5                 | 8.13.4       | `8.13.4` -> `8.17.7` -> `9.4.5`                          | `8.17.7` is the last version in version `8` |
 | 10.1.4                | 8.13.4       | `8.13.4 -> 8.17.7 -> 9.5.10 -> 10.1.4`                   | `8.17.7` is the last version in version `8`, `9.5.10` is the last version in version `9` |
 | 11.3.4                | 8.13.4       | `8.13.4` -> `8.17.7` -> `9.5.10` -> `10.8.7` -> `11.3.4` | `8.17.7` is the last version in version `8`, `9.5.10` is the last version in version `9`, `10.8.7` is the last version in version `10` |
+| 12.0.2                | 11.3.4       | `11.3.4` -> `11.11.x` -> `12.0.2`                        | `11.11.x` is the last version in version `11`
 
 More information about the release procedures can be found in our
 [release documentation](https://gitlab.com/gitlab-org/release/docs). You may also want to read our