Move default `workspaceImage` setting out of experimental

[lucasvaltl]

Is your feature request related to a problem? Please describe

Currently, to set a global default workspace image, you need to add it to a config patch file in the following manner:

experimental:
  webapp:
    server:
      workspaceDefaults:
        # update with the docker image you want for workspaces. If the workspace default
        # base image is stored in a private repo, this registry should be the same as the
        # registry specified in config page in KOTS dashboard. This is the only supported authentication method atm.
        workspaceImage: registry.server.io/repository/workspace-image:latest

Describe the behaviour you'd like

• Default workspace image can be set in the config - happy to chat about where and how exactly.
• We have a guide in our docs that explains how & why to do this and clarifies the limitations

Describe alternatives you've considered

• Expect every user to have a image set in the gitpod.yml - this is a hassle for some users, and something they would have to remember for every project / repo.

Related ticket

• [installer] Move defaultBaseImageRegistryWhitelist out of experimental section #10791 --> it is likely best to tackle this issue and 10791 together

Additional context

• Internal discussion