restore merged-usr behavior in installers (team#40367)

This is mostly because the Technical Committee decided we should stop
supporting non-merged-usr, but also because we *did* find
issues *without* merged-usr. In any case, our old configuration will
stop being supported in bookworm, so let's stop accumulating legacy
here.

howto/new-machine-hetzner-robot.md

@@ -193,7 +193,6 @@ which we'll use below.
         mkdir /target/run/udev && mount -o bind /run/udev /target/run/udev &&
         apt-get install -y grml-debootstrap && \
         grml-debootstrap \
-            --debopt "--no-merged-usr" \
             --grub "$BOOT_DISK" \
             --target /target \
             --hostname `hostname` \

howto/upgrades/bullseye.mdwn

@@ -223,8 +223,13 @@ considering that [merged /usr will be the default in bullseye +
 1](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=978636#178). This, however, can be done *after* the upgrade but needs to be
 done *before* the next major upgrade (Debian 12, bookworm).
 
-The (bullseye) installers should be tweaked to remove the
-`--no-merged-usr` everywhere, in any case. See [ticket 40367](https://gitlab.torproject.org/tpo/tpa/team/-/issues/40367).
+In other words, in the `bookworm` upgrade instructions, we should
+prepare the machines by doing:
+
+    apt install usrmerge
+
+This can also be done at any time after the bullseye upgrade (and can
+even be done in buster, for what that's worth).
 
 ### slapd