fix: declare gson as runtime rather than test (#1004)

Fixing similar issues as googleapis/java-pubsub#1239

Author: suztomo

google-cloud-firestore/pom.xml

@@ -107,6 +107,11 @@
  <groupId>com.google.cloud</groupId>
  <artifactId>google-cloud-core</artifactId>
  </dependency>
+ <dependency>
+ <groupId>com.google.code.gson</groupId>
+ <artifactId>gson</artifactId>
+ <scope>runtime</scope>
+ </dependency>
  <dependency>
  <groupId>com.google.protobuf</groupId>
  <artifactId>protobuf-java-util</artifactId>
@@ -130,11 +135,6 @@
  <version>1.10.19</version>
  <scope>test</scope>
  </dependency>
- <dependency>
- <groupId>com.google.code.gson</groupId>
- <artifactId>gson</artifactId>
- <scope>test</scope>
- </dependency>
  <!-- Need testing utility classes for generated gRPC clients tests -->
  <dependency>
  <groupId>com.google.api</groupId>