Update MSC to match how room versions work

As per 1773.

proposals/1804-advertising-capable-room-versions.md

@@ -3,7 +3,7 @@
 Currently clients need to guess at which room versions the server supports, if any. This is particularly
 difficult to do as it generally requires knowing the state of the ecosystem and what versions are
 available and how keen users are to upgrade their servers and clients. The impossible judgement call
-for when to prompt people to upgrade shouldn't be impossible, or even a judgement call.
+for when to encourage people to upgrade shouldn't be impossible, or even a judgement call.
 
 
 ## Proposal
@@ -16,37 +16,27 @@ this proposal suggests a `m.room_versions` capability be introduced like the fol
 {
     "capabilities": {
         "m.room_versions": {
-            "m.development": ["state-res-v2-test", "event-ids-as-hashes-test", "3"],
+            "default": "1",
-            "m.beta": ["2"],
+            "available": {
-            "m.default": "1",
+                "1": "stable",
-            "m.recommended": "1",
+                "2": "stable",
-            "m.mandatory": ["1"]
+                "state-res-v2-test": "unstable",
+                "event-ids-as-hashes": "unstable",
+                "3": "unstable"
+            }
         }
     }
 }
 ```
 
-The labels picked (`m.development`, etc) are based upon the categories for different room versions.
-Note that `m.default` and `m.recommended` reinforce that there is exactly 1 version in each category.
-
 Clients are encouraged to make use of this capability to determine if the server supports a given
-version, and at what state.
+version, and at what level of stability.
-
-Clients should prompt people with sufficient permissions to perform an upgrade to upgrade their rooms
-to the `m.recommended` room version.
-
-Room versions might appear under multiple categories under some circumstances. In particular, it is
-expected that anything in `m.development` or `m.beta` appears exactly once in the whole capability
-whereas `m.default`, `m.recommended`, and `m.mandatory` may duplicate a room version. The duplication
-is possible due to the definitions of each category:
 
-* `m.default` - This is the room version that the server is going to apply to all new rooms by default.
+The default version is the version that the server is using to create new rooms with. Clients can
-* `m.recommended` - The version clients should be prompting people to upgrade to.
+make the assumption that the default version is a stable version.
-* `m.mandatory` - The version the server is going to enforce on all pre-existing rooms.
 
-With these definitions, it is possible that a room version fits multiple criteria (ie: "please upgrade
+Clients should encourage people with sufficient permissions to perform an upgrade to upgrade their
-your rooms to version X which is also the default for new rooms"). Clients will generally only be
+rooms to the `default` room version when the room is using an `unstable` version.
-interested in the `m.recommended` room version, leaving the rest as informational for users.
 
 
 ## Potential issues