Use c++17 for the json stub compilation

Author: andersfugmann

Changelog.md

@@ -8,7 +8,7 @@
 - Improve copying of comments from .proto files into ocaml code using
   omd to parse markdown
 - Fix problem with deserializing large singed integers (#45)
-- Compile json test stub with C++14 mode
+- Compile json test stub with C++17 mode
 - Fix unrolled annotation
 
 ## 6.1.0: 2024-04-25

test/dune

@@ -35,8 +35,8 @@
  (foreign_stubs
   (language cxx)
   (names protobuf2json)
-  ;; set std=c++14 after standard and before derived c flags
-  (flags (:standard "-std=c++14" (:include c_flags.sexp)) ))
+  ;; set std=c++17 after standard and before derived c flags
+  (flags (:standard "-std=c++17" (:include c_flags.sexp)) ))
  (c_library_flags (:standard (:include c_library_flags.sexp)))
 )