build: align minimum clang dependency with ndn-cxx

ndn-cxx requires clang 6 or later, so there's no point
supporting older versions in ndn-tools

Change-Id: I7be683abd4fce12c85187c0b5abf2ee288bbe908
diff --git a/manpages/conf.py b/manpages/conf.py
index 262a32a..7c3ea95 100644
--- a/manpages/conf.py
+++ b/manpages/conf.py
@@ -18,7 +18,7 @@
 # -- Project information -----------------------------------------------------
 
 project = u'NDN Essential Tools'
-copyright = u'Copyright © 2014-2021 Named Data Networking Project.'
+copyright = u'Copyright © 2014-2022 Named Data Networking Project.'
 author = u'Named Data Networking Project'
 
 # The short X.Y version.