fix: bump min cdk version to 2.28.0 to use node 14 (backport #300)

[mergify]

This is an automatic backport of pull request #300 done by Mergify.
Cherry-pick of 511e534 has failed:

On branch mergify/bp/kubectl-v26/main/pr-300
Your branch is up to date with 'origin/kubectl-v26/main'.

You are currently cherry-picking commit 511e534.
  (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:   .gitattributes
	modified:   .gitignore
	new file:   .gitpod.yml
	modified:   .projen/deps.json
	modified:   .projen/files.json
	modified:   .projenrc.ts
	modified:   CONTRIBUTING.md
	modified:   package.json
	modified:   yarn.lock

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   API.md
	both modified:   test/kubectl-layer.integ.snapshot/lambda-layer-kubectl-integ-stack.assets.json
	both modified:   test/kubectl-layer.integ.snapshot/lambda-layer-kubectl-integ-stack.template.json

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

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