Add Go 1.21, drop 1.19, update default to 1.20

[motoki317]

Summary

I wanted support for Go 1.21, so I looked up recent change patterns and did the following:

• Added support for Go 1.21
• Dropped support for Go 1.19 which will no longer be updated
• Updated default Go version from 1.19 to 1.20

Use Cases

This will enable Go buildpack users to build with Go 1.21.

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: motoki317 / name: motoki317 (934953d, 6bb77ec)

[ForestEckhardt]

This looks great! Thank you.

[motoki317]

Thank you for merging.
@ForestEckhardt Is it a coincidence that the release workflow is failing?
#423

[robdimsdale]

I'm not sure why the tests are failing. I re-triggered it in CI, and I a ran locally also.

Everything passed locally, but failed again in CI. It seems to be a non-deterministic timeout issue which would indicate some sort of environmental issue 🤔

---

Update: it passed on another re-run: https://github.com/paketo-buildpacks/go-dist/actions/runs/5868840655/job/15963496429

Even if the issue is related to go 1.21 I don't think it's worth reverting this change. We would have made this change ourselves anyway as we intend on supporting the versions of go that are supported upstream by google.

I do think it's worth us digging into why it seems more flakey on github runners vs locally.

[ForestEckhardt]

I feel like this is a broader issue that I have seen and I would love to see someone try and tackle this issue paketo-buildpacks/packit#473