Add `site.feed.updated = latest_post` setting

[Krinkle]

Two main uses cases:

1. Reduce deployment churn in repositories that hold a docsite in addition to source code. These currently regenerate and find something new commit on every change, even when nothing in the site has been changed. The only file changing each time is feed.xml. For example: https://github.com/qunitjs/qunit/commits/gh-pages

2. Improve reproducibility of the build, as highlighted via https://github.com/jekyll/jekyll/pull/7187, by offering a choice that simply eliminates use of current time entirely, not even having to mock it.

[parkr]

Boolean config options are always restrictive. What if we went with something that allowed the user to decide from one of a list of values? Like feed.updated: latest_post would choose the publish time of the latest post, and feed.updated: site_time (default) would be site.time. Something like that, the values could change but it would grant more freedom in the future.

[parkr]

Cc @vincerubinetti and https://github.com/jekyll/jekyll-feed/issues/387