Update Ubuntu/Debian install instructions

Unfortunately the Erlang Solutions package repository for Ubuntu and Debian packages has not been updated for recent Ubuntu releases or the latest LTS release. Simplify the instructions to use `apt-get` by default, while still providing an option for those on older releases to use the Erlang Solutions packages.

Fixes elixir-lang#1575.

install.markdown

@@ -65,10 +65,14 @@ If your distribution contains an old Elixir/Erlang version, see the sections bel
     * Run: `eopkg install elixir`
 
   - **Ubuntu** or **Debian**
-    * Add Erlang Solutions repository: `wget https://packages.erlang-solutions.com/erlang-solutions_2.0_all.deb && sudo dpkg -i erlang-solutions_2.0_all.deb`
-    * Run: `sudo apt-get update`
-    * Install the Erlang/OTP platform and all of its applications: `sudo apt-get install esl-erlang`
-    * Install Elixir: `sudo apt-get install elixir`
+    * From primary package repositories:
+      * Run: `sudo apt-get install elixir`
+
+    * From Erlang Solutions, for more recent Elixir/Erlang versions on Ubuntu LTS (< 22.04) or Debian Stable releases:
+      * Add Erlang Solutions repository: `wget https://packages.erlang-solutions.com/erlang-solutions_2.0_all.deb && sudo dpkg -i erlang-solutions_2.0_all.deb`
+      * Run: `sudo apt-get update`
+      * Install the Erlang/OTP platform and all of its applications: `sudo apt-get install esl-erlang`
+      * Install Elixir: `sudo apt-get install elixir`
 
   - **Void Linux**
     * Run: `xbps-install -S elixir`