Atom feed entries have a mandatory "updated" time field. However, it is
possible to have tags but no releases, so the current mechanism fails to
give an "updated" field then.

Instead, let's always use the associated commit and its commit time to
generate the "update" time field.

Note that entries can also have an optional "published" field, but this
is not always needed and other Atom feeds (e.g. Github) also do not set
this field. Also see https://validator.w3.org/feed/docs/atom.html.

Closes #325607