Move Gateway compatibility tests to smoke-test
We currently install `@apollo/gateway` plus a bunch of its old versions at the top level, for use in a few compatibility tests (which mostly just test if TS compiles). These versions are old enough that they force us to override engine-strict for Node v18, and generally lead to lots of stuff in package-lock.json (including AS3!) that's unnecessary for most work.

And now we're running into issues like needing to teach Renovate to override engine-strict because it's now running on v18.

We now have `smoke-test` as a separate test that does its own `npm install`ing; we should test the old gateway versions there instead.