Switch `signUrl` method in google-cloud-storage to use v4 as default signature version

[houglum]

V2 is discouraged in favor of V4. We should figure out a way to migrate users to use V4, and at some point in the future (TBD), we should make the default signature version V4 instead of V2.

[frankyn]

A migration plan is defined internally but pending v4 being default timeline.

[houglum]

I mostly made this issue so that code TODOs could reference it :)
~~ redacted migration plan link~~ (@frankyn)