x/pkgsite: add "v" and "go" prefixes to versions on pkg.go.dev/vuln pages

[julieqiu]

It isn't valid SEMVER to prefix versions with "v" or "go" (https://semver.org/#is-v123-a-semantic-version), but it would be nice for readability to have this on the pkg.go.dev/vuln pages.

We should add the relevant prefix when the page is rendered on pkgsite.

/cc @golang/vulndb @golang/pkgsite

[gopherbot]

Change https://go.dev/cl/409634 mentions this issue: internal/frontend: add version prefixes