gracefully handle swapping with old F-Droid, which only offers index v0 (index.jar)

Up though v1.14 clients, the Nearby/swap process only published index.jar (aka index v0). %1.15 removes all support for index v0 !1085 , so newer clients won't be able to connect to older clients' swap repos. The client should gracefully handle this, and prompt the user to get the other user to upgrade their F-Droid (it'll be available via that swap session).

Since this arrangement does provide an upgrade path, it doesn't seem worthwhile to maintain v0 support going forward.

Here's a chart of compatibility:

connecting	<=1.14 publish v0	>=1.15 publish v1/v2
<=1.14 read v0/v1	v0	v1
>=1.15 read v1/v2	NONE	v1/2