Use stable MSC3069 `is_guest` flag on `/whoami`. (#12021)

Keeping backwards compatibility with the unstable flag for now.

changelog.d/12021.feature

@@ -0,0 +1 @@
+Support only the stable identifier for [MSC3069](https://github.com/matrix-org/matrix-doc/pull/3069)'s `is_guest` on `/_matrix/client/v3/account/whoami`.

synapse/rest/client/account.py

@@ -883,7 +883,9 @@ class WhoamiRestServlet(RestServlet):
         response = {
             "user_id": requester.user.to_string(),
             # MSC: https://github.com/matrix-org/matrix-doc/pull/3069
+            # Entered spec in Matrix 1.2
             "org.matrix.msc3069.is_guest": bool(requester.is_guest),
+            "is_guest": bool(requester.is_guest),
         }
 
         # Appservices and similar accounts do not have device IDs

tests/rest/client/test_account.py

@@ -486,8 +486,9 @@ class WhoamiTestCase(unittest.HomeserverTestCase):
             {
                 "user_id": user_id,
                 "device_id": device_id,
-                # Unstable until MSC3069 enters spec
+                # MSC3069 entered spec in Matrix 1.2 but maintained compatibility
                 "org.matrix.msc3069.is_guest": False,
+                "is_guest": False,
             },
         )
 
@@ -505,8 +506,9 @@ class WhoamiTestCase(unittest.HomeserverTestCase):
             {
                 "user_id": user_id,
                 "device_id": device_id,
-                # Unstable until MSC3069 enters spec
+                # MSC3069 entered spec in Matrix 1.2 but maintained compatibility
                 "org.matrix.msc3069.is_guest": True,
+                "is_guest": True,
             },
         )
 
@@ -528,8 +530,9 @@ class WhoamiTestCase(unittest.HomeserverTestCase):
             whoami,
             {
                 "user_id": user_id,
-                # Unstable until MSC3069 enters spec
+                # MSC3069 entered spec in Matrix 1.2 but maintained compatibility
                 "org.matrix.msc3069.is_guest": False,
+                "is_guest": False,
             },
         )
         self.assertFalse(hasattr(whoami, "device_id"))