Use beautifulsoup4 instead of lxml for URL previews (#19301)

Use `beautifulsoup4` instead of `lxml` for URL previews. This offers
some nicer APIs when parsing HTML and avoids using `libxml`, [which is
unmaintained](https://gitlab.gnome.org/GNOME/libxml2/-/commit/9c80a89af2fdf4f853892f84e46580f4902658ba).

I haven’t done a full regression against commonly previewed sites, but I
expect this will give similar (or better) results.

beautiulsoup also handles decoding the charset for us, which is less
custom code.

---------

Co-authored-by: Andrew Morgan <andrew@amorgan.xyz>
Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

docs/setup/installation.md

@@ -312,7 +312,7 @@ Installing prerequisites on CentOS or Fedora Linux:
 
 ```sh
 sudo dnf install libtiff-devel libjpeg-devel libzip-devel freetype-devel \
-                 libwebp-devel libxml2-devel libxslt-devel libpq-devel \
+                 libwebp-devel libxslt-devel libpq-devel \
                  python3-virtualenv libffi-devel openssl-devel python3-devel
 sudo dnf group install "Development Tools"
 ```
@@ -638,10 +638,6 @@ This is critical from a security perspective to stop arbitrary Matrix users
 spidering 'internal' URLs on your network. At the very least we recommend that
 your loopback and RFC1918 IP addresses are blacklisted.
 
-This also requires the optional `lxml` python dependency to be  installed. This
-in turn requires the `libxml2` library to be available - on  Debian/Ubuntu this
-means `apt-get install libxml2-dev`, or equivalent for your OS.
-
 ### Backups
 
 Don't forget to take [backups](../usage/administration/backups.md) of your new server!