Bug 1782295: Rename private window separation pref. r=pbz

I don't see any reason to keep this under the same namespace as the other former `privacySegmentation` prefs - but perhaps I'm missing something here.

Depends on D156733

Differential Revision: https://phabricator.services.mozilla.com/D156734

toolkit/components/messaging-system/lib/SpecialMessageActions.jsm

@@ -183,7 +183,7 @@ const SpecialMessageActions = {
     const allowedPrefs = [
       "browser.dataFeatureRecommendations.enabled",
       "browser.startup.homepage",
-      "browser.privacySegmentation.windowSeparation.enabled",
+      "browser.privateWindowSeparation.enabled",
       "browser.firefox-view.feature-tour",
     ];
 

toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/index.md

@@ -275,7 +275,7 @@ Action for setting various browser prefs
 Prefs that can be changed with this action are:
 
 - `browser.dataFeatureRecommendations.enabled`
-- `browser.privacySegmentation.windowSeparation.enabled`
+- `browser.privateWindowSeparation.enabled`
 - `browser.startup.homepage`
 
 * args: