[opam] Fix opam file version

opam/opam

 opam-version: "2.0"
 name: "frama-c"
 synopsis: "Platform dedicated to the analysis of source code written in C"
-version: "23.0"
+version: "23.1"
 description:"""
 Frama-C gathers several analysis techniques in a single collaborative
 framework, based on analyzers (called "plug-ins") that can build upon the