Update .gitmodules: use tags spack-stack-2.0.0 for submodules spack and spack-packages (#1862)

Author: climbfuji

.gitmodules

@@ -1,8 +1,8 @@
 [submodule "spack"]
   path = spack
   url = https://github.com/jcsda/spack
-  branch = release/2.0
+  branch = spack-stack-2.0.0
 [submodule "repos/builtin"]
   path = repos/builtin
   url = https://github.com/jcsda/spack-packages
-  branch = release/2.0
+  branch = spack-stack-2.0.0