Update CI to ubuntu-24.04 and bump supported Elixir/OTP versions

The CI was broken because ubuntu-18.04 runners were removed by GitHub.
Update to ubuntu-24.04 with the oldest versions supported by setup-beam
(OTP 24.3+, Elixir 1.11+), bump actions/checkout to v4, and rename
erlef/setup-elixir to erlef/setup-beam.

Author: ericmj

.github/workflows/main.yml

@@ -5,36 +5,36 @@ on: [push, pull_request]
 jobs:
   test:
     name: Test
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-24.04
     strategy:
       fail-fast: false
       matrix:
         include:
-          - erlang: "24.1.7"
-            elixir: "1.13.0"
+          - erlang: "27.3"
+            elixir: "1.19.5"
             lint: true
             big: true
-          - erlang: "24.1.7"
+          - erlang: "27.3"
+            elixir: "1.18.4"
+          - erlang: "27.3"
+            elixir: "1.17.3"
+          - erlang: "26.2"
+            elixir: "1.16.3"
+          - erlang: "26.2"
+            elixir: "1.15.8"
+          - erlang: "25.3"
+            elixir: "1.14.5"
+          - erlang: "25.3"
+            elixir: "1.13.4"
+          - erlang: "24.3.4"
             elixir: "1.12.3"
-          - erlang: "23.3.4"
+          - erlang: "24.3.4"
             elixir: "1.11.4"
-          - erlang: "23.3.4"
-            elixir: "1.10.4"
-          - erlang: "22.3.4"
-            elixir: "1.9.4"
-          - erlang: "22.3.4"
-            elixir: "1.8.2"
-          - erlang: "21.3.8"
-            elixir: "1.7.4"
-          - erlang: "20.3.8"
-            elixir: "1.6.6"
-          - erlang: "19.3.6"
-            elixir: "1.5.3"
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v4
 
       - name: Install OTP and Elixir
-        uses: erlef/setup-elixir@v1
+        uses: erlef/setup-beam@v1
         with:
           otp-version: ${{matrix.erlang}}
           elixir-version: ${{matrix.elixir}}

config/mix_config.exs

@@ -8,10 +8,9 @@ defmodule HexSolver.MixProject do
     [
       app: :hex_solver,
       version: @version,
-      elixir: "~> 1.5",
+      elixir: "~> 1.11",
       start_permanent: Mix.env() == :prod,
       elixirc_paths: elixirc_paths(Mix.env()),
-      config_path: config_path(),
       deps: deps(),
 
       # Hex
@@ -37,14 +36,6 @@ defmodule HexSolver.MixProject do
   defp elixirc_paths(:test), do: ["lib", "test/support"]
   defp elixirc_paths(_), do: ["lib"]
 
-  defp config_path() do
-    if Version.compare(System.version(), "1.11.0") in [:eq, :gt] do
-      "config/config.exs"
-    else
-      "config/mix_config.exs"
-    end
-  end
-
   # Run "mix help deps" to learn about dependencies.
   defp deps do
     [