Update google-cloud-firestore dependency to 0.81.0-beta (GoogleCloudPlatform#1345)

* Update google-cloud-firestore dependency to 0.81.0-beta

* Fix testUpdateServerTimestamp. Directly Use getDate() method  instead of
making a map first.

Author: jlara310

firestore/pom.xml

@@ -45,7 +45,7 @@
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-firestore</artifactId>
-      <version>0.70.0-beta</version>
+      <version>0.81.0-beta</version>
     </dependency>
     <!-- [END fs-maven] -->
 

firestore/src/test/java/com/example/firestore/snippets/ManageDataSnippetsIT.java

@@ -125,8 +125,8 @@ public void testUpdateNestedFields() throws Exception {
   public void testUpdateServerTimestamp() throws Exception {
     manageDataSnippets.updateServerTimestamp();
     DocumentReference docRef = db.collection("objects").document("some-id");
-    Map<String, Object> data = getDocumentDataAsMap(docRef);
-    assertTrue(data.get("timestamp") instanceof Date);
+    DocumentSnapshot data = getDocumentData(docRef);
+    assertTrue(data.getDate("timestamp") instanceof Date);
   }
 
   @Test