Replace @inferred tests with Jet

[mhauru]

@willtebbutt brought up that Jet does much more stringent type stability tests than @inferred, and we should probably switch to using it. If I understood correctly, this requires dropping 1.6 support.

[yebai]

It is a good idea to test more type stability and allocations (#2320); perhaps DynamicPPL is a better place to host these tests?