Fix the critical rayleigh time step calculation in cfd-dem

[acdaigneault]

Description

The critical Rayleigh time step was not properly implemented in CFD-DEM, even tho it was fixed in the past for DEM (see #776 and #939)

Solution

Changed to the same calculation as done in DEM.

Testing

Unresolved CFD-DEM application tests were modified according to the new ratio.

Documentation

Miscellaneous (will be removed when merged)

Checklist (will be removed when merged)

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Code related list:

• [x] All in-code documentation related to this PR is up to date (Doxygen format)
• [x] Lethe documentation is up to date
• [x] Fix has unit test(s) (preferred) or application test(s), and restart files are in the generator folder
• [x] The branch is rebased onto master
• [x] Changelog (CHANGELOG.md) is up to date
• [x] Code is indented with indent-all and .prm files (examples and tests) with prm-indent

Pull request related list:

• [x] Labels are applied
• [x] There are at least 2 reviewers (or 1 if small feature) excluding the responsible for the merge
• [x] If this PR closes an issue or is related to a project, it is linked in the "Projects" or "Development" section
• [x] If the fix is temporary, an issue is opened
• [x] The PR description is cleaned and ready for merge