release/v2.31.0: updating version numbers

RELEASE-NOTES.txt

@@ -5,6 +5,12 @@ Note: these are the release notes for the stan-dev/stan repository.
 Further changes may arise at the interface level (stan-dev/{rstan,
 pystan, cmdstan}) and math library level (stan-dev/math).
 
+v2.31.0 (21 November 2022)
+======================================================================
+- RapidJSON library is now a part of Stan (previously part of CmdStan) (#3143)
+- Added a #define flag flag to enable nested autodiff in model_base class (#3144)
+- Set up the service function so interfaces can offer Laplace approximation. (#3148)
+
 v2.30.0 (1 July 2022)
 ======================================================================
 - Added `const noexcept` modifiers to the three accessors related to step-size (nominal, current, and jitter). (3124)