Add .NET 9 as target framework, fine-tune dependabot, update CI and clean tests removing .NET 6/7 from target frameworks

[64J0]

Description

With this PR,

• I'm adding .NET 9 as a target framework;
• Changing fantomas CI to run using ubuntu-latest;
• Changing build-and-test push trigger to execute only for pushes to the master branch;
• Update dependabot configuration to scan multiple directories;
• Clean test dependencies and remove .NET 6 and 7 from target frameworks.

How to test

Check CI.

[nojaf]

It it a bit weird that you install all these framework, to not have them in a matrix.

[64J0]

I think that for this specific workflow, it makes more sense to have all the used .NET versions available at the same runner process. Other than this, it would trigger too many jobs. Maybe we can test this approach in another PR, what do you think?

[nojaf]

If you care to support the combination of all these dotnet versions on all these operating systems, then a matrix is the way to go.
I wouldn't worry about too many jobs as these will run in parallel anyway.

[64J0]

Indeed, it may be better to get specific problems. I'll open a PR testing this setup later today

[64J0]

#644