Run OTP versions which rely on ancient OpenSSL versions in container

GitHub retired the `ubuntu-20.04` runner image last month. These changes
add a new job which runs the Elixir & OTP pairs which need that Ubuntu
version in a container which uses the official Docker image instead.

A sample error from the existing GitHub workflow job follows:
```
This is a scheduled Ubuntu 20.04 retirement. Ubuntu 20.04 LTS runner
will be removed on 2025-04-15. For more details, see
actions/runner-images#11101
```

Author: sgerrand

.github/workflows/ci.yml

@@ -15,59 +15,9 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - elixir: 1.4.x
-            otp: 20
-            os: ubuntu-20.04
-            gcc: gcc-10
-            gpp: g++-10
-          - elixir: 1.5.x
-            otp: 20
-            os: ubuntu-20.04
-            gcc: gcc-10
-            gpp: g++-10
-          - elixir: 1.6.x
-            otp: 20
-            os: ubuntu-20.04
-            gcc: gcc-10
-            gpp: g++-10
-          - elixir: 1.7.x
-            otp: 20
-            os: ubuntu-20.04
-            gcc: gcc-10
-            gpp: g++-10
-          - elixir: 1.8.x
-            otp: 20
-            os: ubuntu-20.04
-            gcc: gcc-10
-            gpp: g++-10
-          - elixir: 1.9.x
-            otp: 20
-            os: ubuntu-20.04
-            gcc: gcc-10
-            gpp: g++-10
-          - elixir: 1.10.x
-            otp: 21
-            os: ubuntu-20.04
-            gcc: gcc-10
-            gpp: g++-10
-          - elixir: 1.11.x
-            otp: 22
-            os: ubuntu-20.04
-            gcc: gcc-10
-            gpp: g++-10
-          - elixir: 1.11.x
-            otp: 23
-            os: ubuntu-20.04
-            gcc: gcc-10
-            gpp: g++-10
-          - elixir: 1.12.x
-            otp: 23
-            os: ubuntu-20.04
-            gcc: gcc-10
-            gpp: g++-10
           - elixir: 1.13.x
             otp: 24
-            os: ubuntu-20.04
+            os: ubuntu-22.04
             gcc: gcc-10
             gpp: g++-10
           - elixir: 1.14.x
@@ -126,6 +76,66 @@ jobs:
         env:
           CC: ${{matrix.gcc}}
           CXX: ${{matrix.gpp}}
+  mix_test_even_older:
+    name: mix test (Elixir ${{matrix.elixir}} | OTP ${{matrix.otp}})
+    runs-on: ubuntu-latest
+    container:
+      image: ubuntu:focal-20250404
+      env:
+        ImageOS: ubuntu20
+        MIX_ENV: test
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - elixir: 1.4.x
+            otp: 20
+          - elixir: 1.5.x
+            otp: 20
+          - elixir: 1.6.x
+            otp: 20
+          - elixir: 1.7.x
+            otp: 20
+          - elixir: 1.8.x
+            otp: 20
+          - elixir: 1.9.x
+            otp: 20
+          - elixir: 1.10.x
+            otp: 21
+          - elixir: 1.11.x
+            otp: 22
+          - elixir: 1.11.x
+            otp: 23
+          - elixir: 1.12.x
+            otp: 23
+    steps:
+      # Needed by erlef/setup-beam action
+      - run: |
+          apt update -qqy
+          apt install -qqy build-essential libssl-dev unzip
+      - uses: actions/checkout@v2
+      - uses: erlef/setup-beam@v1
+        with:
+          otp-version: ${{matrix.otp}}
+          elixir-version: ${{matrix.elixir}}
+      - name: Install Dependencies
+        run: |
+          mix local.hex --force
+          mix local.rebar --force
+          mix deps.get --only test
+      - name: Cache build artifacts
+        uses: actions/cache@v3
+        with:
+          path: |
+            ~/.hex
+            ~/.mix
+            _build
+          key: ${{ matrix.otp }}-${{ matrix.elixir }}-build
+      - run: mix compile
+        env:
+          CC: gcc-9
+          CXX: g++-9
+      - run: mix test
   dialyzer:
     name: mix dialyzer
     runs-on: ubuntu-latest