AtlasCMakeConfig: set CMP0058 and CMP0074 to OLD

Setting the above two policies to "OLD" silences the following two warnings (see also ATLINFR-2651):

For ninja builds:

CMake Warning (dev):
  Policy CMP0058 is not set: Ninja requires custom command byproducts to be
  explicit.  Run "cmake --help-policy CMP0058" for policy details.  Use the
  cmake_policy command to set the policy and suppress this warning.  This project specifies custom command DEPENDS on files in the build tree
  that are not specified as the OUTPUT or BYPRODUCTS of any
  add_custom_command or add_custom_target:   Control/DataModelTest/DataModelTestDataCommon/CMakeFiles/DataModelTestDataCommonDict.stamp.txt
   Control/DataModelTest/DataModelTestDataCommon/CMakeFiles/DataModelTestDataCommonSerCnv.stamp.txt  For compatibility with versions of CMake that did not have the BYPRODUCTS
  option, CMake is generating phony rules for such files to convince 'ninja'
  to build.  Project authors should add the missing BYPRODUCTS or OUTPUT options to the
  custom commands that produce these files.

When using cmake 3.12:

CMake Warning (dev) at /build2/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.1/InstallArea/x86_64-centos7-gcc8-opt/cmake/modules/AtlasDictionaryFunctions.cmake:655 (find_package):
  Policy CMP0074 is not set: find_package uses _ROOT variables.
  Run "cmake --help-policy CMP0074" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

  CMake variable ROOT_ROOT is set to:

    /cvmfs/sft.cern.ch/lcg/releases/LCG_94/ROOT/6.14.04/x86_64-centos7-gcc8-opt

  For compatibility, CMake is ignoring the variable.