iperf: Update version from 3.1.3 to 3.11

Update download URL to the one mentioned in the GitHub README:

> Downloads of iperf3 are available at:
>
>     https://downloads.es.net/pub/iperf/

iperf.be0

@@ -1,6 +1,6 @@
 #!/usr/bin/env beesh
 
-# BEE_VERSION iperf-3.1.3-0
+# BEE_VERSION iperf-3.11-0
 
 ## this file was created by bee init and should be executed to build a
 ## bee-package. (Additional hints are located at the end of this file.)
@@ -9,7 +9,7 @@
 ## The source URL(s) define the location of the sources that will be
 ## downloaded. Version variables may be used to simplify reuse of this bee-file.
 
-SRCURL[0]="https://iperf.fr/download/source/iperf-${PKGVERSION}-source.tar.gz"
+SRCURL[0]="https://downloads.es.net/pub/iperf/iperf-${PKGVERSION}.tar.gz"
 
 ###############################################################################
 ## Add URLs/pathes to patch files to the PATCHURL array.