Adopt Slurm 6.5.6 to workaround long hostname issues in Debian 12

[tpdownes]

Adopt Slurm 6.5.6 which builds upon 6.5.5 to extend long hostname workaround to dynamic nodes, including login nodes. See GoogleCloudPlatform/slurm-gcp#142 for fuller description of changes.

Submission Checklist

Please take the following actions before submitting this pull request.

• Fork your PR branch from the Toolkit "develop" branch (not main)
• Test all changes with pre-commit in a local branch #
• Confirm that "make tests" passes all tests
• Add or modify unit tests to cover code changes
• Ensure that unit test coverage remains above 80%
• Update all applicable documentation
• Follow Cloud HPC Toolkit Contribution guidelines #

[tpdownes]

All integration tests passed on the draft PR. 1 automated retry was necessary on the Lustre example.