x/pkgsite: show "Added in go.1.x" for each std package as a whole

[adonovan]

For std packages, pkgsite shows "Added in go1.x" for each symbol that was not present when the package was created, but there is no indication of which version the Go package itself was created.

See also:

• x/pkgsite: missing added in label for type in runtime/cgo package #58853

[gabyhelp]

Similar Issues

• x/pkgsite: show the stdlib/module version when a symbol was added #42933
• x/pkgsite: show the version when a symbol was added to a package #37102
• x/pkgsite: "added in version" does not account for new packages #51013

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[adonovan]

Dup of #51013