Fix clirr errors (#2269)

Author: wu-hui

grpc-google-cloud-firestore-v1/clirr-ignored-differences.xml

@@ -1,4 +1,9 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- see http://www.mojohaus.org/clirr-maven-plugin/examples/ignored-differences.html -->
 <differences>
+  <difference>
+    <differenceType>7012</differenceType>
+    <className>com/google/firestore/v1/FirestoreGrpc$AsyncService</className>
+    <method>* executePipeline(*)</method>
+  </difference>
 </differences>

proto-google-cloud-firestore-v1/clirr-ignored-differences.xml

@@ -223,4 +223,9 @@
     <method>* setUnknownFields(*)</method>
     <to>**</to>
   </difference>
+  <difference>
+    <differenceType>7012</differenceType>
+    <className>com/google/firestore/v1/ValueOrBuilder</className>
+    <method>*</method>
+  </difference>
 </differences>