Use unstable prefix for 3PID unbind API

changelog.d/5980.feature

@@ -1 +1 @@
-Add POST /_matrix/client/r0/account/3pid/unbind endpoint from MSC2140 for unbinding a 3PID from an identity server without removing it from the homeserver user account.
+Add POST /_matrix/client/unstable/account/3pid/unbind endpoint from MSC2140 for unbinding a 3PID from an identity server without removing it from the homeserver user account.

changelog.d/6062.bugfix

@@ -0,0 +1 @@
+Add POST /_matrix/client/unstable/account/3pid/unbind endpoint from MSC2140 for unbinding a 3PID from an identity server without removing it from the homeserver user account.

synapse/rest/client/v2_alpha/account.py

@@ -529,7 +529,7 @@ def on_POST(self, request):
 
 
 class ThreepidUnbindRestServlet(RestServlet):
-    PATTERNS = client_patterns("/account/3pid/unbind$")
+    PATTERNS = client_patterns("/account/3pid/unbind$", releases=(), unstable=True)
 
     def __init__(self, hs):
         super(ThreepidUnbindRestServlet, self).__init__()