v2021: Import latest upstream changes
Sync with Apertis 0.2021.2 to import the upstream changes.
Pull 44aa78d2 "Include flatpak metadata in OSTree commits" in particular to fix errors when trying to use on v2022 flatpaks generated on v2021:
Error: Commit metadata for runtime/org.apertis.mildenhall.Platform/x86_64/v2021 not matching expected metadata