Fix CSI migration for vSphere volumes

Restore `migratedPlugins` entry for in-tree vSphere volumes and mark them
as migrated. Otherwise the CSI volume plugin ignores in-tree vSphere
volumes and they will never get attached / mounted by CSI migration.

The entry in `migratedPlugins` was incorrectly removed during
CSIMigrationvSphere feature gate removal.

pkg/volume/csi/csi_plugin.go

@@ -236,6 +236,9 @@ func (p *csiPlugin) Init(host volume.VolumeHost) error {
 		csitranslationplugins.AzureFileInTreePluginName: func() bool {
 			return utilfeature.DefaultFeatureGate.Enabled(features.CSIMigrationAzureFile)
 		},
+		csitranslationplugins.VSphereInTreePluginName: func() bool {
+			return true
+		},
 		csitranslationplugins.PortworxVolumePluginName: func() bool {
 			return utilfeature.DefaultFeatureGate.Enabled(features.CSIMigrationPortworx)
 		},