chore(deps): the libraries BOM 15.0.0 (#251)

* chore(deps): libraires-bom 13.4.0

#235 is stale

* chore(deps): libraries-bom 13.4.0

* linkage checker and shared deps BOM

* the libraries BOM 13.3.0 (1 release behind)

* Libraries BOM 15.0.0

Author: suztomo

samples/snippets/pom.xml

@@ -30,7 +30,7 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>libraries-bom</artifactId>
-        <version>12.0.0</version>
+        <version>15.0.0</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>