Call out @migrate as a khan-specific directive
Call out @migrate as a khan-specific directive

This audit moves the `@migrate` directive to a separate, commented
section so that it's clear that the directive is khan-specific. At some
point we'd like to discover khan-specific directives automatically.

Issue: see rKLefcb6f0fd2

Test plan:
ka-lint services/*/*.graphql in KA root

Auditors: csilvers

Khan/khan-linter,Khan/khan-linter,Khan/khan-linter,Khan/khan-linter