Ensure smooth transition from old default editor (theia or che-code/insiders) to new default che-code/latest

When updating from an older version of DS 3.x in which theia was the default editor, or from 3.4 where che-code/insiders was the default editor, we need something in place to allow old workspaces and old CheCluster objects to Just Work even though che-code/insiders and che-theia/latest editors are no longer part of DS as of 3.6.

May require documentation about manual update steps (or a bash script + yaml patch), eg.,

• user guide: old devfiles/devworkspaces created before 3.5 need to be updated to use che-code/latest
• admin guide: old CheCluster objects need to be updated to use defaultEditor = che-incubator/che-code/latest

This may include:
1) adding an ability to change default editor in CheCluster during the upgrade to DS 3.6 (automatically? migration script to apply yaml transformation?)

spec:
  devEnvironments:
    defaultEditor: che-incubator/che-code/latest

Also want to update to default to the embedded open-vsx registry (not the public one - see CRW-4117

spec:
  components:
    pluginRegistry:
      openVSXURL: ''

And, since headerMessage might be changed between releases, we should ensure the latest deprecation/removal warning is applied in the CheCluster too - see PR408 for CRW-3663

spec:
  components:
    dashboard:
      headerMessage:
        show: "true"
        text: "Microsoft Visual Studio Code - Open Source is the default <a href='https://access.redhat.com/documentation/en-us/red_hat_openshift_dev_spaces/3.6/html-single/user_guide/index#selecting-a-workspace-ide'>editor</a> for new workspaces. Eclipse Theia is <a href='https://access.redhat.com/documentation/en-us/red_hat_openshift_dev_spaces/3.6/html-single/release_notes_and_known_issues/index#removed-functionalities'>no longer supported</a>."

2)

And there is "runningLimit" field in CR:

spec:
  components:
    devWorkspace:
      runningLimit: '100'

replaced with "maxNumberOfRunningWorkspacesPerUser", starting from DS 3.5.0 (https://doc.crds.dev/github.com/eclipse-che/che-operator/org.eclipse.che/CheCluster/v2@7.60.0, CRW-3870)

spec:
  devEnvironments:
    maxNumberOfRunningWorkspacesPerUser: 100

3)

And there is the problem that the default UDI image is hardcoded in the CRD, which means:

• freshmaker updates to CSV won't change the hardcoded value – CVE fixes in UDI won't propagate to CheCluster
• if I install DS 3.4 and don't manually fix my CheCluster to use a newer UDI image (from the latest in the CRD or CSV) I'll always open empty workspaces with the udi-rhel8:3.4 image, not the :3.6 one.

  spec:
    devEnvironments:
        defaultComponents:
          - container:
              image: >-
                registry.redhat.io/devspaces/udi-rhel8@sha256:5d17d145844b4e6632aba478c7c66c6c5925cbb64ecf86b2c7083b2dfea6dfcb
  

This problem could also break people expecting new versions of content in the UDI image, like dotnet 7 or a newer python or node installation. "But the release notes say it's updated – why does my empty workspace not have it?"

4) dealing with existing che-theia workspaces after an upgrade to DS 3.6, as they won't be able to run – do we convert them?