Bump linkage-checker-enforcer-rules from 1.5.3 to 1.5.7 (#371)

Bumps [linkage-checker-enforcer-rules](https://github.com/GoogleCloudPlatform/cloud-opensource-java) from 1.5.3 to 1.5.7.
- [Release notes](https://github.com/GoogleCloudPlatform/cloud-opensource-java/releases)
- [Changelog](https://github.com/GoogleCloudPlatform/cloud-opensource-java/blob/master/CHANGELOG.md)
- [Commits](https://github.com/GoogleCloudPlatform/cloud-opensource-java/commits)

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spring-cloud-gcp-dependencies/pom.xml

@@ -362,7 +362,7 @@
 							<dependency>
 								<groupId>com.google.cloud.tools</groupId>
 								<artifactId>linkage-checker-enforcer-rules</artifactId>
-								<version>1.5.3</version>
+								<version>1.5.7</version>
 							</dependency>
 						</dependencies>
 						<executions>