Bump golang to v1.21 (backport #684)

[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 #684 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of dd16a95 has failed:

On branch mergify/bp/v1.2/pr-684
Your branch is ahead of 'origin/v1.2' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit dd16a95.
  (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:   pkg/config/coerce.go
	modified:   pkg/config/cos_test.go
	modified:   pkg/config/rename.go
	modified:   pkg/console/console.go
	modified:   pkg/console/dashboard_panels.go
	modified:   pkg/console/helper.go
	modified:   pkg/console/util_test.go
	modified:   pkg/console/webhooks_test.go
	modified:   pkg/util/disk.go
	modified:   pkg/util/disk_test.go
	modified:   pkg/widgets/dropdown.go
	modified:   pkg/widgets/select.go
	modified:   pkg/widgets/util.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   Dockerfile.dapper
	both modified:   pkg/console/install_panels.go
	both modified:   pkg/console/util.go
	deleted by us:   pkg/preflight/checks.go
	deleted by us:   pkg/preflight/checks_test.go
	both modified:   pkg/widgets/panel.go

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

[Vicente-Cheng]

lgtm, thanks!