chore: align Elixir requirement and CI versions with dependencies

- Require Elixir ~> 1.15 in mix.exs (needed by plug 1.20)
- Use OTP 27.3 and Elixir 1.18.4 in CI and Publish workflows to match .tool-versions

Author: JeremyAube

.github/workflows/ci.yml

@@ -7,8 +7,8 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        otp-version: [27.0]
-        elixir-version: [1.17.1]
+        otp-version: [27.3]
+        elixir-version: [1.18.4]
 
     env:
       MIX_ENV: test

.github/workflows/publish.yml

@@ -14,8 +14,8 @@ jobs:
       - uses: actions/checkout@v4
       - uses: erlef/setup-beam@v1
         with:
-          otp-version: 27.x
-          elixir-version: 1.18.x
+          otp-version: 27.3
+          elixir-version: 1.18.4
       - run: sudo apt-get update
       - run: sudo apt-get install -y libvips-dev
       - run: vips --vips-version

mix.exs

@@ -7,7 +7,7 @@ defmodule PlugImageProcessing.Mixfile do
     [
       app: :plug_image_processing,
       version: @version,
-      elixir: "~> 1.13",
+      elixir: "~> 1.15",
       package: package(),
       elixirc_paths: elixirc_paths(Mix.env()),
       compilers: Mix.compilers(),