Add full support for latest version of Go (1.21)

[percivalalb]

I encountered an issue when running go-lsif in the official docker image over a repo using go 1.21 which makes use of the newly introduced slices stdlib package:

<package>/<file>.go:7:2: package slices is not in GOROOT (/usr/local/go/src/slices)

I believe the docker image is shipped with 1.18 which is missing the new packages added in

• go1.21: https://tip.golang.org/doc/go1.21#library
• go1.20: https://tip.golang.org/doc/go1.20#library

1. Therefore the version of go in the docker image needs bumping.
2. The code generation script for https://github.com/sourcegraph/lsif-go/blob/master/internal/gomod/stdlib.go#L8 would also need re-running to include the new packages in this list.

This is by no means a complete list of what need doing, just a couple of the points I spotted with my limited knowledge of how go-lsif works.