chore: Post 3.9 version bump.

configure.ac

@@ -24,7 +24,7 @@
 # file for complete information.
 
 # Initialize the autoconf system for the specified tool, version and mailing list
-AC_INIT(iperf, 3.9, https://github.com/esnet/iperf, iperf, https://software.es.net/iperf/)
+AC_INIT(iperf, 3.9+, https://github.com/esnet/iperf, iperf, https://software.es.net/iperf/)
 m4_include([config/ax_check_openssl.m4])
 m4_include([config/iperf_config_static_bin.m4])
 AC_LANG(C)