[dev.typeparams] src,cmd: bump go.mod to 'go 1.18'

Necessary for building/testing generics code within src/ or src/cmd/
since CL 332373, and we'll need to do this eventually anyway.

Change-Id: Ia8c658c92d861fd3803fa18bfc80407c3381b411
Reviewed-on: https://go-review.googlesource.com/c/go/+/332554


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>