Change default coarse grid solver for the geometric multigrid

[lpsaavedra]

Description

This PR makes the direct solver the default coarse grid solver for the geometric multigrid alternatives in the matrix-free application.

Testing

All the existent tests pass with this coarse-grid solver. The parameter files were cleaned and the coarse grid solver section removed.

Documentation

The default value was updated in the linear solver section of the documentation.

Checklist (will be removed when merged)

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Code related list:

• All in-code documentation related to this PR is up to date (Doxygen format)
• Copyright headers are present and up to date
• Lethe documentation is up to date
• The branch is rebased onto master
• Code is indented with indent-all and .prm files (examples and tests) with prm-indent
• If parameters are modified, the tests and the documentation of examples are up to date
• Changelog (CHANGELOG.md) is up to date if the refactor affects the user experience or the codebase

Pull request related list:

• No other PR is open related to this refactoring
• Labels are applied
• There are at least 2 reviewers (or 1 if small feature) excluding the responsible for the merge
• The PR description is cleaned and ready for merge

[blaisb]

Perfect, that's a very healthy change I think. Direct is the way to go 90% of the time

[blaisb]

@lpsaavedra can you add a changelog entry for this since this is a modification to a default parameter?