More cleanup before upgrading to OpenAPI 3.1 (#1455)

* `cross_signing_key.yaml`: the parameter documentation already restricts the number of properties
 * `receipts.yaml`: use `maxProperties: 0` to say the object is empty (the comment is still there but is not really needed any more)
 
Signed-off-by: Alexey Rusakov <Kitsune.Ral@users.sf.net>

changelogs/client_server/newsfragments/1455.clarification

@@ -0,0 +1 @@
+Fix various typos throughout the specification.

data/api/client-server/definitions/cross_signing_key.yaml

@@ -35,6 +35,8 @@ properties:
       The public key.  The object must have exactly one property, whose name is
       in the form `<algorithm>:<unpadded_base64_public_key>`, and whose value
       is the unpadded base64 public key.
+    minProperties: 1
+    maxProperties: 1
     example:
       "ed25519:alice+base64+public+key": "alice+base64+public+key"
   signatures:

data/api/client-server/receipts.yaml

@@ -93,7 +93,8 @@ paths:
             application/json: {
               }
           schema:
-            type: object  # empty json object
+            type: object
+            maxProperties: 0  # empty json object
         429:
           description: This request was rate-limited.
           schema: