x/pkgsite: Update go version on docs

[Stavrospanakakis]

What is the URL of the page with the issue?

https://github.com/golang/pkgsite/blob/master/doc/design.md
https://github.com/golang/pkgsite/blob/master/CONTRIBUTING.md

What is your user agent?

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.5060.66 Safari/537.36

Screenshot

What did you do?

What did you expect to see?

I expected to see Go 1.19 as stated in README.md

What did you see instead?

I saw Go 1.13 and Go 1.18

I'd love to open a pull request and fix the issue :)

[gopherbot]

Change https://go.dev/cl/416114 mentions this issue: doc,CONTRIBUTING.md: update go version to 1.19