Remove stale references to C++14.

The main branch was upgraded to use C++17 after the 29.0 branch cut, in commit
https://github.com/protocolbuffers/protobuf/commit/fe535930d3183b46d7e088683cbe8c49285715f8

There are still stale references to C++14 in the code, build chain, and
READMEs. This commit cleans up the stale configs and settings.

PiperOrigin-RevId: 706000867