[DiffRedesign] Desired State Compare - pick the versions in the header #7828

Part of #7655.

Scope

Desired State Compare only.

• The two versions in the page header become selects, so you can compare other versions without going back to the Desired State page.
• Picking a version updates the URL and reloads the diff. The selected resource and filters stay when they still apply.

Depends on

• [DiffRedesign] Resource list with the diff next to it #7658, which adds the header with both versions.

Design: section "Compare page" (G).