Change of default branch name of spack submodule

[climbfuji]

Just now (04/10/2024) we renamed the default branch of the spack submodule from jcsda_emc_spack_stack to spack-stack-dev. This was discussed and agreed upon several months ago at the spack-stack developer meeting. This new name is (a) shorter, (b) matches better how we name the tags of the spack submodule (spack-stack-1.7.0 for example), and (c) also reflects that there are other contributors to our spack fork than JCSDA and EMC: namely, EPIC, NASA GMAO, NRL.

There were only three pull open requests that were all automatically updated, and so was the branch protection rule.

PR #1072 updates the branch name in the .gitmodules file.

Users with existing clones of spack-stack that are actively used need to run git remote update in the spack submodule/subdirectory and the top-level spack-stack directory to refresh the git submodule configuration.