Draft: [Debian 12] Prepare proposed 12.2 update

• d/gbp.conf: Configure for bookworm branch
• Backport merged-/usr support changes from trixie:
  • Implement merged-/usr by post-merging. This changes the bootstrap order so that it will be possible for a future version of base-files in trixie/sid to take responsibility for creating the /bin, /sbin, /lib* symlinks, and is a prerequisite for lifting the moratorium imposed by #1035831. (Closes: #1049898; implementation by Helmut Grohne)
  • functions: Reinstate setup_merged_usr() as it existed before fixing #1049898, for backwards-compatibility with older versions of mmdebstrap.
  • functions: Default to merged-/usr for suites newer than bookworm, even for the buildd profile. This allows packages targeting trixie to assume that merged-/usr is the only layout possible, and is a prerequisite for lifting the moratorium imposed by #1035831.
  • Mention --merged-usr in --help output. (Closes: #1031828)
• Backport autopkgtest regression fixes from trixie:
  • Revert fake/schroot-1.6.10-3 to an accurate emulation of the behaviour of schroot 1.6.10-3 (Closes: #983311)
  • Document schroot 1.6.10-3 with the default profile as expected to fail to use /dev/pts in a lxc >= 3 or Docker container; this is a schroot bug (#983423) and not a debootstrap bug
  • With these changes, the autopkgtest is confirmed to pass in autopkgtest-virt-qemu (Closes: #983197)