feat!: remove deprecated usage of URIs for referencing simple extensions

[benbellick]

Closes #970

BREAKING CHANGE: dropping URI usage from protobufs in favor of URNs

[benbellick]

Please see the issue for some discussion on the change. In summary, it has been 5 months and the most commonly used libraries all support urns, so I think it is time to complete the migration :)

[nielspardon]

LGTM, should start also removing even less recent deprecations

[vbarua]

I think this is safe to remove now. The only thing I would advocate for is commenting out any of the ... [deprecated = true]; lines to make sure we avoid re-using field numbers, but I also don't feel strongly about it.

[benbellick]

Oh yeah, I should have added those as reserved. I will do that in a subsequent commit soon!

[benbellick]

I have updated those fields to be reserved.

One other thing to call out. The test files still use URIs to reference other extensions. Though I think it is reasonable to tackle that in a subsequent PR.

[vbarua]

Sweet, using reserved is perfect for this. That said, let's hold off on merging this. We have some other changes in flight this week, and I think this changes should be in its own release.

On Friday, I will manually trigger a release to flush pending changes out. Then I will merge this change and release it by itself.