Update minimum cmake version from 3.0 to 3.6

With cmake 4 [1] the support for cmake 3.5 or lower was cut [2]
cmake 4 was deployed on github.com around 2025-04-02
which started to break many things e.g. ogg
cmake 3.5 was released in 2016 [3] and many versions in the
3.30 range has warned about the deprecation

[1] https://cmake.org/download/#latest

[2] https://cmake.org/cmake/help/v4.0/release/4.0.html

[3] https://www.kitware.com/cmake-3-5-0-available-for-download/

Note that the PR is similar to
ogg!29