x/pkgsite: handle packages from an archived repo specially

[jba]

If a repo has been archived (as described for GItHub here), we should reduce its search rank, and add an indicator to its pages.

[myitcv]

Somewhat related to #40357

[BenjamenMeyer]

This might be easier than #41126 (comment) since GitHub allows a repository to be marked as an archive, the integration with GitHub should be able to detect that the owner archived it.
Indeed, the GetRepository API call to GitHub (https://docs.github.com/en/rest/repos/repos#get-a-repository) has an archived attribute on the result, which also shows up in https://docs.github.com/en/rest/repos/repos#list-organization-repositories.

Seems like the mechanics for updating the data in pkg.go.dev should be able to easily detect this flag being set and put up a notice like GitHub does regarding archived repositories: