Remove support for Libxc 2
Description
This removes support for Libxc 2. The last bugfix release of Libxc 2 is four years old now and Libxc 5 will be released soon. Also mark Libxc 3 as deprecated, as Libxc 4 it's already two years and a half old.
News snippet
Remove support for Libxc 2 and mark Libxc 3 as deprecated.
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.
Closes #269 (closed)