RC release should use beta for api version (#5695)

php/ext/google/protobuf/package.xml

@@ -17,8 +17,8 @@
   <api>3.7.0</api>
  </version>
  <stability>
-  <release>stable</release>
-  <api>stable</api>
+  <release>beta</release>
+  <api>beta</api>
  </stability>
  <license uri="https://opensource.org/licenses/BSD-3-Clause">3-Clause BSD License</license>
  <notes>GA release.</notes>