feat: Update to v3

[peterbourgon]

Follow-on from #4183

If your repo has a tag that Go modules parses as semver (like v2.11.1) then the module name in the corresponding go.mod must meet Go modules' expectations, specifically if it is v2 or above it must include a /vN version suffix. If you don't do this, your project is essentially unusable by consumers.

After discussion in #4183 I've bumped the version to v3 and made the necessary changes to import paths. If and when this merges to main, you'll need to tag v3.0.0 for it to all work properly.

Fixes #2602

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[alexec]

@peterbourgon we may be making some big changes soon and we may well want to fix this problem. Can we close this for the moment, and then I'd like to get you involved in that work?

[peterbourgon]

Happy to advise.

[alexec]

@peterbourgon I think we're essentially green-lighted for v3. Do you want to complete this PR to help drive this forward? I can see several build jobs need fixing.

[peterbourgon]

I don't have enough domain knowledge of the project to make meaningful progress on those. I'm happy to advise whoever can drive the work about modules-related errata.