Feed Validation Error: RSS 2.0 instead Atom

Hi,
I have a small problem with the feed for publication dates that are not correctly detected in [Liferea](https://lzone.de/liferea), the 'news reader' I currently use .
Searching on 'Jekyll' doc and for [Jekyll Feed plugin](https://github.com/jekyll/jekyll-feed#why-atom-and-not-rss) it seems that [feed.xml](https://github.com/abseil/abseil.github.io/blob/b4ae05707eafef75dba6364d9de506d5610950aa/feed.xml) is classified as 'RSS 2.0' (`rss version="2.0"`) but coded as 'Atom'.
For the validation I also used the [W3C Validator site](https://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Ffeeds.feedburner.com%2Fabseilio).
Having the ['jekyll/jekyll-feed' feed.xml](https://github.com/jekyll/jekyll-feed/blob/master/lib/jekyll-feed/feed.xml) as reference I did some local tests by changing some xml fields to get the validation and I came to have an acceptable situation (only some validation warning) with the attached changes.
I attach the changes here as not knowing the site and all its information are unable to build a dedicated pull request with all correct data for the feed.

Thanks for your work.
Enrico

_`feed-atom-test.patch`_
```
diff --git a/feed.xml b/feed.xml
index 8e7a351..aa36233 100644
--- a/feed.xml
+++ b/feed.xml
@@ -2,22 +2,27 @@
layout: null
---

-
-
+
Abseil Blog & Tips
- An open-source collection of C++ library code designed to
- augment the C++ standard library
- {{ site.url }}
+ An open-source collection of C++ library code designed to
+ augment the C++ standard library
+ {{ site.time | date_to_xmlschema }}
+ {{ site.url | xml_escape }}
{% for post in site.posts %}
{% unless post.draft %}
-
- {{ post.title | xml_escape }}
- {{ post.content | xml_escape }}
- {{ post.date | date_to_xmlschema }}
- {{ post.url | prepend: site.url }}
- {{ post.url | prepend: site.url }}
-
+
+ {{ post.title | xml_escape }}
+ {{ post.content | xml_escape }}
+ {{ post.date | date_to_xmlschema }}
+ {{ post.date | date_to_xmlschema }}
+
+
+ {{ post_author_name | default: "???" | xml_escape }}
+
+
+ {{ post.id | absolute_url | xml_escape }}
+
{% endunless %}
{% endfor %}
-
-
\ No newline at end of file
+
+

```