Drop libxml2 from apt install so it works on newer Ubuntu

> [!CAUTION]
> This is preliminary since I can't test it on my Mac. But some
research and AI suggested this was the best solution, so I'm curious to
get feedback.

A student reported that the rbenv dependency install fails on both
Windows (WSL) and Ubuntu:

  sudo apt install -y build-essential tklib zlib1g-dev libssl-dev \
    libffi-dev libxml2 libxml2-dev libxslt1-dev libreadline-dev libyaml-dev

libxml2 2.14 changed its library version, so Ubuntu 25.10 and later
renamed the runtime package from libxml2 to libxml2-16. On those
releases `libxml2` no longer exists and apt aborts the whole command.

The feedback suggested replacing libxml2 with libxml2-16, but that
package doesn't exist on Ubuntu 22.04/24.04, so it would break the
command for students on those versions. Instead, drop the explicit
runtime package: libxml2-dev depends on the right runtime library on
each release (libxml2 on older Ubuntu, libxml2-16 on newer), so apt
_should_ install it automatically. Or so I'm told.

This is preliminary: it hasn't been tested on Ubuntu yet (written on
macOS). Ideally, we would test on 24.04 and 26.04/WSL before merging.
But no idea how we do that

Author: ajdubovoy

_partials/cn/rbenv.md

@@ -14,7 +14,7 @@ rm -rf ~/.rbenv
 然后在终端里运行:
 
 ```bash
-sudo apt install -y build-essential tklib zlib1g-dev libssl-dev libffi-dev libxml2 libxml2-dev libxslt1-dev libreadline-dev libyaml-dev
+sudo apt install -y build-essential tklib zlib1g-dev libssl-dev libffi-dev libxml2-dev libxslt1-dev libreadline-dev libyaml-dev
 ```
 
 ```bash

_partials/es/rbenv.md

@@ -14,7 +14,7 @@ rm -rf ~/.rbenv
 Luego ejecuta lo siguiente en tu terminal:
 
 ```bash
-sudo apt install -y build-essential tklib zlib1g-dev libssl-dev libffi-dev libxml2 libxml2-dev libxslt1-dev libreadline-dev libyaml-dev
+sudo apt install -y build-essential tklib zlib1g-dev libssl-dev libffi-dev libxml2-dev libxslt1-dev libreadline-dev libyaml-dev
 ```
 
 ```bash

_partials/fr/rbenv.md

@@ -14,7 +14,7 @@ rm -rf ~/.rbenv
 Puis dans le terminal, exécute :
 
 ```bash
-sudo apt install -y build-essential tklib zlib1g-dev libssl-dev libffi-dev libxml2 libxml2-dev libxslt1-dev libreadline-dev libyaml-dev
+sudo apt install -y build-essential tklib zlib1g-dev libssl-dev libffi-dev libxml2-dev libxslt1-dev libreadline-dev libyaml-dev
 ```
 
 ```bash

_partials/pt/rbenv.md

@@ -14,7 +14,7 @@ rm -rf ~/.rbenv
 Então no terminal, execute:
 
 ```bash
-sudo apt install -y build-essential tklib zlib1g-dev libssl-dev libffi-dev libxml2 libxml2-dev libxslt1-dev libreadline-dev libyaml-dev
+sudo apt install -y build-essential tklib zlib1g-dev libssl-dev libffi-dev libxml2-dev libxslt1-dev libreadline-dev libyaml-dev
 ```
 
 ```bash

_partials/rbenv.md

@@ -14,7 +14,7 @@ rm -rf ~/.rbenv
 Then in the terminal, run:
 
 ```bash
-sudo apt install -y build-essential tklib zlib1g-dev libssl-dev libffi-dev libxml2 libxml2-dev libxslt1-dev libreadline-dev libyaml-dev
+sudo apt install -y build-essential tklib zlib1g-dev libssl-dev libffi-dev libxml2-dev libxslt1-dev libreadline-dev libyaml-dev
 ```
 
 ```bash