Update Go version in README.md (#5471)
Update Go version in README.md (#5471)

Prometheus 2.9 requires Go 1.12+ to build it from source
This commit updates the README file according to this requirement

Signed-off-by: Vandewilly Silva <2d94f60ccfbd3d0dbe2cb2a1fac1a892fbeccba9@hpe.com>
GoogleCloudPlatform/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus,Gouthamve/prometheus,alileza/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,kangwoo/prometheus,keegancsmith/prometheus,prometheus/prometheus,prometheus/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,alileza/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,kangwoo/prometheus,keegancsmith/prometheus,prometheus/prometheus,kangwoo/prometheus,prometheus/prometheus