Update asdf install docs in Debian install guide

Instead of trying to update the version of asdf being used, just point
users to the guide on their website.

Ideally we'd do this for Elixir and Erlang as well, but new versions of
those packages may sometimes have compatibility issues with Akkoma. For
now, update those to the latest OTP and Elixir versions known to be
comaptible with Akkoma.

docs/docs/installation/debian_based_en.md

@@ -35,32 +35,24 @@ sudo useradd -r -s /bin/false -m -d /var/lib/akkoma -U akkoma
 
 ### Install Elixir and Erlang
 
+#### Using `apt`
 If your distribution packages a recent enough version of Elixir, you can install it directly from the distro repositories and skip to the next section of the guide:
 
 ```shell
 sudo apt install elixir erlang-dev erlang-nox
 ```
 
-Otherwise use [asdf](https://github.com/asdf-vm/asdf) to install the latest versions of Elixir and Erlang.
+#### Using `asdf`
+If your distribution does not have a recent version of Elxir in their repositories, you can use [asdf](https://asdf-vm.com/) to install a newer version of Elixir and Erlang.
 
 First, install some dependencies needed to build Elixir and Erlang:
 ```shell
 sudo apt install curl unzip build-essential autoconf m4 libncurses5-dev libssh-dev unixodbc-dev xsltproc libxml2-utils libncurses-dev
 ```
 
-Then login to the `akkoma` user and install asdf:
-```shell
-git clone https://github.com/asdf-vm/asdf.git ~/.asdf --branch v0.11.3
-```
+Then login to the `akkoma` user.
 
-Add the following lines to `~/.bashrc`:
-```shell
-. "$HOME/.asdf/asdf.sh"
-# asdf completions
-. "$HOME/.asdf/completions/asdf.bash"
-```
-
-Restart the shell:
+Install asdf by following steps 1 to 3 on [their website](https://asdf-vm.com/guide/getting-started.html), then restart the shell to load asdf:
 ```shell
 exec $SHELL
 ```
@@ -69,15 +61,15 @@ Next install Erlang:
 ```shell
 asdf plugin add erlang https://github.com/asdf-vm/asdf-erlang.git
 export KERL_CONFIGURE_OPTIONS="--disable-debug --without-javac"
-asdf install erlang 25.3.2.5
-asdf global erlang 25.3.2.5
+asdf install erlang 26.2.5.4
+asdf global erlang 26.2.5.4
 ```
 
 Now install Elixir:
 ```shell
 asdf plugin-add elixir https://github.com/asdf-vm/asdf-elixir.git
-asdf install elixir 1.15.4-otp-25
-asdf global elixir 1.15.4-otp-25
+asdf install elixir 1.17.3-otp-26
+asdf global elixir 1.17.3-otp-26
 ```
 
 Confirm that Elixir is installed correctly by checking the version: