build: switch to latest versions of slf4j and logback

slf4j-1.6.0
logback-0.9.21

pom.xml

@@ -194,12 +194,12 @@
         <dependency>
             <groupId>org.slf4j</groupId>
             <artifactId>slf4j-api</artifactId>
-            <version>1.5.11</version>
+            <version>1.6.0</version>
         </dependency>
         <dependency>
             <groupId>ch.qos.logback</groupId>
             <artifactId>logback-classic</artifactId>
-            <version>0.9.18</version>
+            <version>0.9.21</version>
         </dependency>
 
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