Switch the default for --flatpak-metadata to 'both'
The old default of 'annotations' didn't make sense since flatpak 1.6 and
newer only support labels. Switch the default to 'both' for maximum
compatibility.
Also add --flatpak-metadata option to 'flatpak-module local-build' - it
was documented but missing.
fixup! Switch the default for --flatpak-metadata to 'labels'