Update firestore version as per launch cal (#202)

* bump firestore version so that we can release the right version

* update to 18 as per the launchcal

Author: VinayGuthal

firebase-firestore/gradle.properties

@@ -1,2 +1,2 @@
-version=17.1.6
+version=18.0.0
 latestReleasedVersion=17.1.5