upgrading to sundials v6.0.0; modifying with new version number

Author: wds15

README.md

@@ -26,7 +26,7 @@ Stan Math depends on four libraries:
 
 - Boost (version 1.75.0): [Boost Home Page](https://www.boost.org)
 - Eigen (version 3.3.9: [Eigen Home Page](https://eigen.tuxfamily.org/index.php?title=Main_Page)
-- SUNDIALS (version 5.7.0): [Sundials Home Page](https://computation.llnl.gov/projects/sundials/sundials-software)
+- SUNDIALS (version 6.0.0): [Sundials Home Page](https://computation.llnl.gov/projects/sundials/sundials-software)
 - Intel TBB (version 2020.3): [Intel TBB Home Page](https://www.threadingbuildingblocks.org)
 
 These are distributed under the `lib/` subdirectory. Only these
@@ -96,7 +96,7 @@ The standalone makefile ensures that all the required `-I` include
 statements are given to the compiler and the necessary libraries are
 linked: `~/stan-dev/math` and `~/stan-dev/math/lib/eigen_3.3.9` and
 `~/stan-dev/math/lib/boost_1.75.0` and
-`~/stan-dev/math/lib/sundials_5.7.0/include` and
+`~/stan-dev/math/lib/sundials_6.0.0/include` and
 `~/stan-dev/math/lib/tbb_2020.3/include`. The
 `~/stan-dev/math/lib/tbb` directory is created by the `math-libs`
 makefile target automatically and contains the dynamically loaded

make/libraries

@@ -12,7 +12,7 @@ BOOST ?= $(MATH)lib/boost_1.75.0
 EIGEN ?= $(MATH)lib/eigen_3.3.9
 OPENCL ?= $(MATH)lib/opencl_3.0.0
 TBB ?= $(MATH)lib/tbb_2020.3
-SUNDIALS ?= $(MATH)lib/sundials_5.7.0
+SUNDIALS ?= $(MATH)lib/sundials_6.0.0
 BENCHMARK ?= $(MATH)lib/benchmark_1.5.1
 GTEST ?= $(BENCHMARK)/googletest/googletest
 CPPLINT ?= $(MATH)lib/cpplint_1.4.5