Update the required Go version to 1.12
See
@@ -54,7 +54,7 @@ All our issues are regularly tagged so that you can also filter down the issues
## Dependency management
-The Prometheus project uses [Go modules](https://golang.org/cmd/go/#hdr-Modules__module_versions__and_more) to manage dependencies on external packages. This requires a working Go environment with version 1.11 or greater installed.
+The Prometheus project uses [Go modules](https://golang.org/cmd/go/#hdr-Modules__module_versions__and_more) to manage dependencies on external packages. This requires a working Go environment with version 1.12 or greater installed.
All dependencies are vendored in the `vendor/` directory.
