adapt to latest GTFS-Flex spec

[derhuerst]

As GTFS-Flex v2 is currently a proposal (and in the process of becoming an official GTFS extension, see google/transit#382), there are changes being made that require producers & consumers to adapt.

@leonardehrenfried is currently updating OTP to the current state of the proposal, so bbnavi's GTFS-Flex feed should be adapted accordingly:

• Using stop_areas.txt instead of location_groups.txt google/transit#338

[derhuerst]

The (for now) final GTFS-Flex version that has been voted on is here: google/transit#433
Once this is merged, it will be officially part of the spec.

[leonardehrenfried]

And since summer '23 the location groups have changed twice again!