Merge pull request #2887 from stan-dev/eigen339-compat

Minor return type update for Eigen backwards compatibility

Author: WardBrian

stan/math/prim/fun/eigenvalues_sym.hpp

@@ -21,7 +21,7 @@ namespace math {
  */
 template <typename EigMat, require_eigen_matrix_dynamic_t<EigMat>* = nullptr,
           require_not_st_var<EigMat>* = nullptr>
-Eigen::Vector<value_type_t<EigMat>, -1> eigenvalues_sym(const EigMat& m) {
+Eigen::Matrix<value_type_t<EigMat>, -1, 1> eigenvalues_sym(const EigMat& m) {
   using PlainMat = plain_type_t<EigMat>;
   const PlainMat& m_eval = m;
   check_nonzero_size("eigenvalues_sym", "m", m_eval);