Upgrade slf4j 1.7.21

Author: slandelle

pom.xml

@@ -412,7 +412,7 @@
 		<source.property>1.8</source.property>
 		<target.property>1.8</target.property>
 		<netty.version>4.0.36.Final</netty.version>
-		<slf4j.version>1.7.20</slf4j.version>
+		<slf4j.version>1.7.21</slf4j.version>
 		<logback.version>1.1.7</logback.version>
 		<log4j.version>1.2.17</log4j.version>
 		<testng.version>6.9.9</testng.version>