Force a more recent version of rstan so builds don't fail

Author: VisruthSK

.github/workflows/R-CMD-check.yaml

@@ -44,7 +44,7 @@ jobs:
 
       - uses: r-lib/actions/setup-r-dependencies@v2
         with:
-          extra-packages: any::rcmdcheck, any::BH, any::RcppEigen, any::vdiffr, any::cmdstanr
+          extra-packages: any::rcmdcheck, any::BH, any::RcppEigen, any::vdiffr, any::cmdstanr, any::rstan@>=2.36
           needs: check
 
       - name: Install CmdStan