Rollup merge of #35742 - spladug:readme-cmake-version, r=nikomatsakis
Rollup merge of #35742 - spladug:readme-cmake-version, r=nikomatsakis

Update minimum CMake version in README

The minimum got bumped in the LLVM upgrade of #34743.

graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust