Add 'deprecated:true' to /v1/send_{join,leave} (#1518)

The words already say this is deprecated, but it was missing the flag.

changelogs/server_server/newsfragments/1518.clarification

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

data/api/server-server/joins-v1.yaml

@@ -218,6 +218,7 @@ paths:
 
   "/send_join/{roomId}/{eventId}":
     put:
+      deprecated: true
       summary: Submit a signed join event to a resident server
       description: |-
         **Note:**

data/api/server-server/leaving-v1.yaml

@@ -141,6 +141,7 @@ paths:
             }
   "/send_leave/{roomId}/{eventId}":
     put:
+      deprecated: true
       summary: Submit a signed leave event to a resident server
       description: |-
         **Note:**