Set minimum cmake policy version in FindTBB.cmake

[jspanchu]

The current FindTBB.cmake calls cmake_minimum_required which potentially generates warnings for consumer projects that use cmake >= 4.0

The version check can be scoped within the file instead:

cmake_policy(PUSH)
cmake_policy(VERSION 3.5...3.20) # choose "3.20" based on the highest version known to be otherwise policy warning-free
# ...
# ...

# at the end of the file
cmake_policy(POP)