x/pkgsite: set up automated updating of x-repo dependencies

[findleyr]

Following up on #62031, where pkg.go.dev was broken due to an old x/mod version, we should set up automatic updating of pkgsite dependencies (at least x repos).

@golang/release what do we need to do to set this up?

[dmitshur]

Also see #56530, which I think this is either a subset or a duplicate of. It was waiting for the next friction fixit week, but we've discussed and agreed to up its priority, so I'm resuming work on it.

Note another x/pkgsite-specific option is to opt the repository in for automatic monthly tagging by creating its first tag (i.e. v0.1.0 or v1.0.0).

[findleyr]

Thanks, I think we'd rather not start tagging pkgsite if there's no need, so let's close this as a dupe of #56530.