Update Twitter shortcode oEmbed endpoint

The existing endpoint will be retired and removed on November 23, 2021.
References:

- https://twittercommunity.com/t/consolidating-the-oembed-functionality/154690
- https://developer.twitter.com/en/docs/twitter-for-websites/oembed-api#Embedded

This is a backward compatible change.

The existing endpoint requires a single parameter: the id of the tweet.

The new endpoint requires two parameters: the id of the tweet, and the
user with whom it is associated. For the moment, if you supply the wrong
user, the request will be redirected (with a small delay) to the correct
user/id pair. This behavior is undocumented, but we will take advantage
of it as Hugo site authors transition to the new syntax.

{{< tweet 1453110110599868418 >}} --> works, throws warning, deprecate at some point

{{< tweet user="SanDiegoZoo" id="1453110110599868418" >}} --> new syntax

Fixes #8130

docs/content/en/showcase/letsencrypt/index.md

@@ -9,7 +9,7 @@ byline: "[bep](https://github.com/bep), Hugo Lead"
 
 The **Let’s Encrypt website** has a common set of elements: A landing page and some other static info-pages, a document section, a blog, and a documentation section. Having it moved to Hugo was mostly motivated by a _simpler administration and Hugo's [multilingual support](/content-management/multilingual/)_. They already serve HTTPS to more than 60 million domains, and having the documentation available in more languages will increase that reach.[^1]
 
-{{< tweet 971755920639307777 >}}
+{{< tweet user="letsencrypt" id="971755920639307777" >}}
 
 I helped them port the site from Jekyll to Hugo. There are usually very few surprises doing this. I know Hugo very well, but working on sites with a history usually comes up with something new.