Move back to Open MPI 4 on SLES15 at NCCS

This release moves SLES15 runs at NCCS back to Open MPI 4.1.6.

Testing showed that Open MPI 5 caused crashes at C360...though only C360. 🤷🏼

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From CHANGELOG.md

Fixed

• Move back to Open MPI 4 on SLES15 at NCCS
  • Testing showed crashes at C360 and under various other circumstances

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What's Changed

• Move back to Open MPI 4 at NCCS on SLES15 by @mathomp4 in #120

Full Changelog: v4.28.0...v4.28.1