Add systemd to pmOS edge

This adds the systemd repo to pmOS edge, allowing folks to (among other things) build pmOS images with systemd more easily.

Currently we are requiring this to be merged before inviting more folks to test systemd on pmOS (see: #2938 (closed))

Fixes: #2773 (closed)

Replaces: !4956 (closed)

How to review this

NOTE: It's probably not worth reviewing this until all of the TODOs below are done.

There are a lot of commits in this branch/MR, I don't expect everyone to review them all... they have been reviewed when they were merged into the staging branch.

If you want some suggestions about what would be useful to review for this MR, ask me and I'll add it here.

TODO

• Go through !4956 (closed) and make sure that anything still relevant there is added to the staging branch
• Remove "not for master" commits
  • CI: build_changed_aports: systemd staging (MR 5070)
    • Half of this patch can be dropped when this is merged: !5576 (merged)
    • NOTE: drop part of patch that sets the mirror right before merging this branch
  • CI: hardcode get_upstream_branch (MR 5070)
    • NOTE: drop entire patch right before merging this branch
  • CI: disable armhf, riscv64, x86 (MR 5070)
    • NOTE: drop entire patch right before merging this branch
  • remove temp/* pkgs that conflict with systemd repo
    • Depends on build.postmarketos.org#140 (closed)
    • Also see: build.postmarketos.org#143 (closed)
  • temp/mutter-mobile: drop
  • temp/gnome-settings-daemon-mobile: drop
  • CHECK: systemd/xorg-server: new aport
  • CHECK: systemd/networkmanager: new aport
• Resolve linked dependency issues/MRs
• pmbootstrap patch: pmbootstrap!2405 (merged) (needed for bpo to split the repo)
• bpo changes, so this does not get built as part of the regular repository when merged (build.postmarketos.org#140 (closed))
• fix apk-tools /usr merge breaking upgrades: #3202 (closed)
• systemd: revert patches for splitting /usr (#3234)
  • clayton: not required, can be dropped later when the usr merge stuff is sorted out
• merge remaining MRs targeting this branch (that are ready to merge)
  • https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests?scope=all&state=opened&target_branch=master_staging_systemd&draft=no
• add systemd to the issue templates for pmaports
  • !6022 (merged)
• instruct existing systemd users to update their /etc/apk/repositories and add the edge repo below systemd will be done after this is merged and built, moved this todo to build.postmarketos.org#140 (closed)
• test merge and repo_bootstrap locally for all architectures
  • x86_64
  • aarch64
  • armv7
  • x86, armhf: let's skip these, it is likely that they work as well and if not we fix it up right after the merge
• adjust pkgs.postmarketos.org to correctly display systemd packages will be done after this is merged and built, moved this todo to build.postmarketos.org#140 (closed)
• update the motd to make it less scary (currently says apk upgrade will break your system, etc)
  • !6023 (merged)
• make repo_bootstrap happy again (dbus has no bootstrap logic and builds systemd early (in bootstrap=3)
  • !6025 (merged)