This updates the minimal supported version of CMake to 3.16, which in turn enables us to use more CMake features and have a cleaner build system.
This is the version that provides most new features and also the one we use in our latest docker image for CI.
It is now possible to have any alignment restriction on rodata in the user
applicaiton. It will not affect the first section which must be aligned
on a 16-byte bound.
Closes https://github.com/espressif/esp-idf/issues/6719
