Release process: reorder steps.

With the previous order, feature/buster would end up with frozen APT
repositories, which was not intended.

wiki/src/contribute/release_process.mdwn

@@ -289,19 +289,19 @@ Update other base branches
 1. Merge the release branch into `devel` following the instructions for
    [[merging base branches|APT_repository/custom#workflow-merge-main-branch]].
 
-2. Merge `devel` into `feature/buster`, *without* following the instructions for
+2. [[Thaw|APT_repository/time-based snapshots#thaw]], on the devel
+   branch, the time-based APT repository snapshots that were used
+   during the freeze.
+
+3. Merge `devel` into `feature/buster`, *without* following the instructions for
    [[merging base branches|APT_repository/custom#workflow-merge-main-branch]].
    (For now `feature/buster` is handled as any other topic branch
    forked off `devel`: its base branch is set to `devel`.)
 
-3. Ensure that the release, `devel` and `feature/buster` branches
+4. Ensure that the release, `devel` and `feature/buster` branches
    have the expected content in `config/APT_overlays.d/`: e.g. it must
    not list any overlay APT suite that has been merged already.
 
-4. [[Thaw|APT_repository/time-based snapshots#thaw]], on the devel
-   branch, the time-based APT repository snapshots that were used
-   during the freeze.
-
 5. Push the modified branches to Git:
 
         git push origin                          \