Update default buildpack version in AutoDevOps docs

What does this MR do and why?

Update the docs to reflect that the default buildpack was updated to heroku/buildpacks:22: https://gitlab.com/gitlab-org/cluster-integration/auto-build-image/-/blob/b7d54d5ffe89cccdffb19eca0939eeb8df81e981/src/build.sh#L62

References

I ran into this while working on #23706

MR acceptance checklist

It's a tiny docs change.

How to set up and validate locally

  1. Navigate to https://docs.gitlab.com/ee/topics/autodevops/stages.html#auto-build-using-cloud-native-buildpacks
  2. See outdated version
Edited by 🤖 GitLab Bot 🤖

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