[docs]: be explicit about the hosting requirements

[mrsimonemms]

For running Gitpod, it is a requirement that the Kubernetes nodes use Ubuntu as their operating system. This requirement is only obliquely referenced in documentation, whether that's in the guides, the Installer readme or the main Gitpod.io website.

We should have a section describing what your cluster should look like and prominently say that Ubuntu is a requirement. In terms of versions, 18.04 is (currently) the minimum supported version but we recommend 20.04 or above.

My understanding of requirements as of 2022-01-06 (this should not be treated as definitive and may be superseded when this ticket is picked up)

• Ubuntu 18.04 (20.04 or above recommended)
• Kubernetes 1.21 or above (there is a validation test for this). It will run on 1.20, but the ttlSecondsAfterFinished on jobs won't be respected until 1.21
• MySQL 5.7

[mrsimonemms]

Now included on https://www.gitpod.io/docs/self-hosted/latest/cluster-set-up#node-and-container-requirements