ci: replacing Linkage Monitor with Linkage Checker enforcer rule

Linkage Monitor was comparing the dependencies with the latest
Libraires BOM. As the Libraries BOM synchronizes the content with
the Google Cloud BOM, we no longer need Linkage Monitor.

Linkage Checker enforcer rule is more appropriate to find
dependency conflicts among the libraries listed in the BOM.

pom.xml

@@ -78,4 +78,56 @@
       </dependency>
     </dependencies>
   </dependencyManagement>
+
+  <profiles>
+    <profile>
+      <id>linkage-check</id>
+      <build>
+        <plugins>
+          <plugin>
+            <groupId>org.apache.maven.plugins</groupId>
+            <artifactId>maven-enforcer-plugin</artifactId>
+            <version>3.0.0-M3</version>
+            <dependencies>
+              <dependency>
+                <groupId>com.google.cloud.tools</groupId>
+                <artifactId>linkage-checker-enforcer-rules</artifactId>
+                <version>1.5.12</version>
+              </dependency>
+              <dependency>
+                <groupId>org.codehaus.mojo</groupId>
+                <artifactId>extra-enforcer-rules</artifactId>
+                <version>1.3</version>
+                <exclusions>
+                  <exclusion>
+                    <groupId>org.eclipse.aether</groupId>
+                    <artifactId>aether-util</artifactId>
+                  </exclusion>
+                </exclusions>
+                <scope>compile</scope>
+              </dependency>
+            </dependencies>
+            <executions>
+              <execution>
+                <id>enforce-linkage-checker</id>
+                <!-- Important! Should run after compile -->
+                <phase>verify</phase>
+                <goals>
+                  <goal>enforce</goal>
+                </goals>
+                <configuration>
+                  <rules>
+                    <LinkageCheckerRule
+                      implementation="com.google.cloud.tools.dependencies.enforcer.LinkageCheckerRule">
+                      <dependencySection>DEPENDENCY_MANAGEMENT</dependencySection>
+                    </LinkageCheckerRule>
+                  </rules>
+                </configuration>
+              </execution>
+            </executions>
+          </plugin>
+        </plugins>
+      </build>
+    </profile>
+  </profiles>
 </project>