doc: fix knot version in doxygen

Doxyfile

@@ -38,8 +38,7 @@ PROJECT_NAME           = Knot
 # could be handy for archiving the generated documentation or if some version
 # control system is used.
 
-PROJECT_NUMBER         = 1.6
-
+PROJECT_NUMBER         = 2.3.0
 # Using the PROJECT_BRIEF tag one can provide an optional one line description
 # for a project that appears at the top of each page and should give viewer a
 # quick idea about the purpose of the project. Keep the description short.