Require the minimum Gradle version necessary to support lazy task configuration

[benjamin-bader]

I feel like I read somewhere that 4.9 and above are required for proper lazy task configuration, but the javadoc for the relevant APIs that I've seen so far say 4.0 or 4.3.

Whatever the case may be, we'll need to drop support for older Gradle versions.

We have compatibility hacks going back to Gradle 2ish, and it will be great to get rid of them.

[benjamin-bader]

Turns out this is actually 5.1. I'll eventually want full task-configuration avoidance, and the full form of those APIs apparently didn't exist until then: https://docs.gradle.org/current/userguide/task_configuration_avoidance.html

This'll require a major version bump - our first.