Bump golang to v1.21 (backport #73)

[mergify]

Problem:
Go v1.20 is EOL on 06 Feb 2024.

Solution:
Bump golang to v1.21.

Related Issue:
harvester/harvester#5310, harvester/harvester#5612

---

This is an automatic backport of pull request #73 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of f4ef139 has failed:

On branch mergify/bp/v1.2/pr-73
Your branch is up to date with 'origin/v1.2'.

You are currently cherry-picking commit f4ef139.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   .golangci.json
	modified:   Dockerfile.dapper
	modified:   go.mod

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   go.sum

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally