feat: Rename GtfsPickupDropOff.ALLOWED to GtfsPickupDropOff.REGULAR

[cswilson252]

Summary:

Renames GtfsPickupDropOff.ALLOWED to GtfsPickupDropOff.REGULAR in the Pickup/Dropoff enum + respective test files.
Fixes #1942

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[qcdyx]

LGTM

[jcpitre]

Tested with this dataset: tld-4477-202603010112.zip on the web validator staging page.
Just confirmed that the forbidden_pickup_type notice is still there.