GitHub Actions workflow improvements (#6)

This PR refactors the existing CI workflow. Notable changes include:

- renaming the file to `ci.yml` to better reflect its function,
- replaces much of the setup-related job steps with
[CargoSense/setup-elixir-project](https://github.com/CargoSense/setup-elixir-project)
reusable Action,
- adds `workflow_dispatch` as an event trigger so that we may manually
invoke the Action (should that prove useful)

~~I've retained the use of a matrix even though there's only one
configured OTP and Elixir version. @billylanchantin and I have been
chatting about what an improved test matrix might look like, but haven't
yet come to a conclusion. One we do, I'm happy to update the matrix with
additional versions.~~

Updated in c9e6068. Pulling out my commit message since it includes some
relevant notes:

> This required using an older version of Ubuntu to support older
Erlang/OTP versions. Pulled that bit of knowledge from the README here:
>
> https://github.com/erlef/setup-beam
> 
> Additionally, Erlang/OTP compatibility charts are maintained here:
> 
>
https://hexdocs.pm/elixir/compatibility-and-deprecations.html#between-elixir-and-erlang-otp
> 
> …and lastly, the matrix expansion via `include` is documented here:
> 
>
https://docs.github.com/en/actions/using-jobs/using-a-matrix-for-your-jobs#expanding-or-adding-matrix-configurations
> 
> I'm _mostly_ confident this will work as desired.

.github/workflows/ci.yml

@@ -0,0 +1,34 @@
+name: CI
+
+on:
+  push:
+    branches: ["main"]
+  pull_request:
+    branches: ["main"]
+  workflow_dipatch:
+
+jobs:
+  test:
+    name: OTP ${{ matrix.otp }} / Elixir ${{ matrix.elixir }}
+    runs-on: ubuntu-20.04
+    env:
+      MIX_ENV: test
+    strategy:
+      fail-fast: false
+      matrix:
+        elixir: ["1.16", "1.15", "1.14", "1.13"]
+        include:
+          - otp: [26, 25, 24]
+          - elixir: "1.14"
+            otp: [25, 24, 23]
+          - elixir: "1.13"
+            otp: [24, 23, 22]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: CargoSense/setup-elixir-project@v1
+        with:
+          otp-version: ${{ matrix.otp }}
+          elixir-version: ${{ matrix.elixir }}
+          build-flags: "--warnings-as-errors"
+      - run: mix format --check-formatted
+      - run: mix test