[CMake] Intel OneAPI MKL

[RMeli]

This PR removes the custom FindMKL.cmake module in favor of using the MKLConfig.cmake provided by Intel OneAPI MKL (since version 2021.3).

intel-mkl is now deprecated in Spack (see spack/spack#41488), and the last release is 2020.4.30.

We recently moved to Intel OneAPI MKL in DLA-Future (see eth-cscs/DLA-Future#1073), which broke sirius+dlaf since they both use MKL_FOUND and other equally named variables, but then use sirius::mkl and MKL::MKL, respectively. This leads to

    179      The link interface of target "DLAF::LAPACK" contains:
     180    
     181        MKL::MKL
     182    
     183      but the target was not found.  Possible reasons include:

(Thanks @albestro for the help spotting that sharing MKL_FOUND was the root of the problem.)

[simonpintarelli]

Thank you @RMeli , lgtm.