Renaming `geosx` into `geos`

Hello guys, since it was decided to rename geosx into geos, it’s now time to take the appropriate actions and reflect the change in our code and organisations!

I’ve listed below what I could come accros. Please feel free to update the list so we’re sure we’re forgetting nothing.
Of course, when I write "Change ...", it means that we have to deal with the aftermath of the change as well (modifications of the scripts, tell our partners, etc).

I did not think about the best order to do all the actions yet. We need to sort this out and organize ourselves.

Regarding github

• Change the github organisation from GEOSX to GEOS GEOS-DEV
• Change the github repository from GEOSX to GEOS

While github provides forwarding from old name to new name, our jenkins/gitlab/travis will need to be updated accordingly.

Regarding other organisations

• Change the readthedocs organisation and project (be careful, the url is hard coded in the C++ code).
• Change the GCP bucket names that contain geosx.

• Change the travis organisation and repositories.
• Change the geosx.org website.
• Update the deployment folders that often contain geosx.
• Update the dockerhub organisation.

Regarding source code

• Change the occurrences of geosx in the C++ namespace, GEOSX_ log/error macros, include guards...
  Done in https://github.com/GEOS-DEV/GEOS/pull/2392
• Change the last occurrences of geosx in the C++ in the variable names, class names, files... Be careful of the uncrustify issues.
• Do the same for other code (not C++), like our cmake code or our travis anchors.
• Rewrite the "How does GEOSX relate to GEOS?" question!
  Done in https://github.com/GEOS-DEV/GEOS/pull/2575
• Change the cmake variables to GEOS_, update doc accordingly.
• Rename geosxats.
• Rename pygeosx.

Regarding the geosx executable

• Deploy both geosx and geos executables to let the end users adapt their scripts.
  PR https://github.com/GEOS-DEV/GEOS/pull/2466
• Documentation should not mention geosx anymore.
  Lot of work first done in https://github.com/GEOS-DEV/GEOS/pull/2392
• Update ATS that uses geosx and not geos.
• Add a deprecation warning when running geosx instead of geos.
• Remove the original geosx executable.

Anything else?

This name change will cost us some time 🤣

Use the unchecked checklist items as the scope, starting with the remaining C++ names and files, CMake variables, deployment folders, GCP buckets, ATS references, and Docker Hub organisation. Review the linked pull requests and existing rename work first; done means the remaining geosx occurrences and listed external integrations are updated without breaking scripts, documentation, or executable compatibility.