Deprecate namespace strategies other than "per user"

[skabashnyuk]

Is your task related to a problem? Please describe.

When switching to DevWorkspace we would like to drop support for namespace strategies other than "per user".
One and only one namespace per user simplifies a lot the design of the workspaces.
We could assume then that every user configuration lives in his namespace.

Describe the solution you'd like

At the "deprecation" stage we need to reflect that fact on documentation and add a warning when someone wants to use it.

Describe alternatives you've considered

Remove support without "deprecation" stage

Additional context

n/a

[skabashnyuk]

There is a "deprecation plan"

• At "Step One" we want to deprecate all namespace strategies other than "per user".
  That means only placeholder or would be available for namespace strategies.

• At "Step One" we will add a warning with the notice about depreciation.

• At "Step One" we will provide some guides of what to do in case if someone is using namespace strategies other than "per user".
  Unfortunately, they would be looks like - save all and create a new cluster.

• At "Step Two" only "per user" namespace strategy would be available.

• At "Step Two" those who have namespace strategies other than "per user" upgrade to the new version would be blocked in some form.