Bump servlet and hotspot client_java deps to 0.0.21

Author: brian-brazil

jmx_prometheus_httpserver/pom.xml

@@ -23,7 +23,7 @@
     <dependency>
       <groupId>io.prometheus</groupId>
       <artifactId>simpleclient_servlet</artifactId>
-      <version>0.0.16</version>
+      <version>0.0.21</version>
     </dependency>
     <dependency>
       <groupId>org.eclipse.jetty</groupId>

jmx_prometheus_javaagent/pom.xml

@@ -23,12 +23,12 @@
     <dependency>
       <groupId>io.prometheus</groupId>
       <artifactId>simpleclient_hotspot</artifactId>
-      <version>0.0.8</version>
+      <version>0.0.21</version>
     </dependency>
     <dependency>
       <groupId>io.prometheus</groupId>
       <artifactId>simpleclient_servlet</artifactId>
-      <version>0.0.8</version>
+      <version>0.0.21</version>
     </dependency>
     <dependency>
       <groupId>org.eclipse.jetty</groupId>