Update to CMake>=3.16

[emhl]

v2.8.12 has been deprecated since the release of cmake v3.19 in 2020. so installing this package with cmake v3.19 or newer installed does not work.

i've tested building the application with cmake v4.0 after this modification

[emhl]

i've done a bit more testing. and building the c++ library from source cargo test --workspace --all-features does not work (but with the prebuild library it seems fine).
but with any modern system, that has a cmake version of at least v3.19, this currently does not work either

[Luthaf]

so installing this package with cmake v3.19 or newer installed does not work.

It should work fine with cmake 3.19+, it's only cmake 4.0 which dropped support for compatiblity with 2.8.

The build from source will require a new release of the C++ library. I'm slowly working toward it, but I have a hard time freeing time for chemfiles these days.

[emhl]

Ah, yes that makes sense. depreciation doesn't mean support is actually dropped. Since the issue currently only exists on cmake 4.0 it probably makes more sense if those users set a different minimal cmake Version Through the CMAKE_POLICY_VERSION_MINIMUM environment variable. instead of breaking the build from source for everyone else.