Bump jetty-server.version from 12.0.20 to 12.0.21 (prometheus#1349)

Bumps `jetty-server.version` from 12.0.20 to 12.0.21.
Updates `org.eclipse.jetty:jetty-server` from 12.0.20 to 12.0.21

Updates `org.eclipse.jetty.ee10:jetty-ee10-servlet` from 12.0.20 to
12.0.21

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integration-tests/it-exporter/it-exporter-servlet-jetty-sample/pom.xml

@@ -16,7 +16,7 @@
     Jetty Sample for the Exporter Integration Test
   </description>
   <properties>
-    <jetty-server.version>12.0.20</jetty-server.version>
+    <jetty-server.version>12.0.21</jetty-server.version>
     <java.version>17</java.version>
   </properties>