Deprecate pangeo-forge-recipes<0.10.0 support

[cisaacstern]

Not a super high priority, but noting that supporting 0.9.4 adds a lot of CI overhead, and branching logic in the code. I would be very surprised to learn that anyone is actively deploying recipes written with <0.10.0 with pangeo-forge-runner, but as good practice probably best to do a deprecation cycle over the course of a few releases?

[moradology]

I'd be in favor of asking forgiveness rather than making things bulletproof. Perhaps just gut the relevant portions and add an exception that very clearly and very quickly alerts users to the need to upgrade?

Work in this direction on #170