Adapt CODEOWNERS file

[RobPasMue]

We might have to create a team of admins for this repo and add the pyansys-ci-bot to it :) as well as use the team for the CODEOWNERS file

Or directly use @ansys-internal/pyansys-core - which is a valid option too.

We should also include specific admins for different sections of the code (for example, any modification to the filetransfer tool should include @greschd's approval). This can be done again from the CODEOWNERS file