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Bump `cmake_minimum_required` to 3.5.

Delete conditional branches exclusive to older versions.

Notable dependents:
  - github.com/grpc/grpc >= 3.5.1
  - github.com/abseil/abseil-cpp >= 3.5
  - github.com/googleapis/google-cloud-cpp >= 3.5

On the other hand, github.com/protocolbuffers/protobuf is >= 3.1.3, but it only depends on GoogleTest 1.10.

Fixes #3523

PiperOrigin-RevId: 392073834