Use (new) API submission by default

[willdurand]

We'd like to use the new-ish API submission by default, and keep the current one (considered "legacy") behind a flag, which we'll remove in v9 (which is the major after the current major).

[willdurand]

cc @eviljeff

[eviljeff]

#2503 - for any outstanding blockers to making it the default.

[willdurand]

In fact, we are not going to make the submission API the default with a way to keep using the old API via sign-addon. We are going to make the submission API the only signing API in v8. Users who want to use the "old" API (sign-addon) should continue to use v7.