x/devtools: Update mojo profile to version 6
x/devtools: Update mojo profile to version 6

This new version includes Dart Mojom Types, so that Dart clients can
use the v23proxy. (Support for Dart servers will be added later.)

Tested the new profile on syncbase, discovery, and v23proxy repos.
(v23proxy needed to lowercase some of its mojom struct fields.)

MultiPart: 1/2

Change-Id: Iee7a77b2e64e27efaf46af711cb96e5df0e39e5e

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools