x/pkgsite: provide documentation for older, still supported go versions

[gnojus]

What is the URL of the page with the issue?

https://pkg.go.dev/slices?tab=versions

What is your user agent?

Screenshot

No response

What did you do?

Visited https://pkg.go.dev/slices?tab=versions to look for docs of the slices package with go version 1.20.

What did you see happen?

No go 1.20 versions in the version list.

What did you expect to see?

Go version 1.20 as an option, because go 1.20 is a supported release.

[aarzilli]

The slices packages is new in go 1.21: https://go.dev/doc/go1.21#slices

[gnojus]

Oops, confused with the previous versions in x/exp.
Thanks