Update cmake_minimum_required to 3.14

[winterheart]

Recent CMake 4.1 warns that <3.10 support will be deprecated. Update minimum version and simplify some parts of project.

Fix python warnings about regexp escape sequences.

[jgm]

Why 3.14 and not 3.10?

[winterheart]

Hi. I've chosen 3.14 to get rid CMP0092 policy declaration and unify find_package(Python3) usage.

[jgm]

I don't have a good sense for the value of retaining compatibility with cmake 3.10 - 3.13. If there might be some value, then I think a bit of extra complexity may be justified. @nwellnhof any thoughts?

[nwellnhof]

Apart from Ubuntu 18.04 which ships with CMake 3.10, it shouldn't matter.

[winterheart]

Ubuntu 18.04 reached EOL in 2023, oldest supported Ubuntu release is 22.04 (2027), where cmake 3.22 is available.

[nwellnhof]

There's long-term support for Ubuntu, but 18.04 seems old enough. Ubuntu 20.04 on the other hand is still widely used, for example in OSS-Fuzz, even if it's out of standard support. It comes with CMake 3.16, so that's the minimum we should support for a while.