Check single-to-multi root mode migration in Che-Theia

[azatsarynnyy]

Is your task related to a problem? Please describe.

ATM it's not fully clear what will be the Che-Theia behavior when the multi-root mode will be enabled for an existing workspace, after upgrading to Che 7.29.0.

Describe the solution you'd like

We need to validate the Che-Theia behavior for the following use case:

• create a new persisted workspace with several projects where the multi-root mode is off
• turn the multi-root mode on and restart the workspace
• make sure the explorer shows a correct projects structure

Describe alternatives you've considered

Additional context

[azatsarynnyy]

I started a workspace from the following Devfile:

apiVersion: 1.0.0
metadata:
  name: che-theia-all
projects:
  - name: che-theia
    source:
      location: 'https://github.com/eclipse-che/che-theia.git'
      type: git
  - name: theia
    source:
      location: 'https://github.com/eclipse-theia/theia.git'
      type: git
components:
  - id: eclipse/che-theia/latest
    type: cheEditor

Note, eclipse/che-theia/latest has the multi-root mode off, by default.

The project explorer displayed the projects' structure correctly:

Then, I switched to eclipse/che-theia/next (where the multi-root mode is on by default) and restarted the workspace.
Now, the project explorer is empty:

We need to make the multi-root mode off by default while investigating this problem.

[azatsarynnyy]

The follow-up issue #19551