Change old-GLIBC Docker image to Rocky Linux 8

[aplowman]

Will fix: hpcflow/hpcflow-new#853

I've tested the build-executables and test workflows in the hpcflow rockylinux branch here:

• https://github.com/hpcflow/hpcflow-new/actions/runs/16303998548/job/46045520023
• https://github.com/hpcflow/hpcflow-new/actions/runs/16304370768/job/46046822893

Unrelated failures:

• the Windows build-executables failure is unrelated (Pytest issue on Windows Pyinstaller-built executable tests hpcflow-new#854)
• I discovered som integration tests run via the built executables also fail, but this is also unrelated (Failing integration tests with Pyinstaller-built executables hpcflow-new#855)

Note: the rockylinux branch in hpcflow-new can be deleted after this PR is merged.