Modernize CI infrastructure

- Update actions to their latest version;
- Update compiler versions to be future proof;
- Set up a periodic build;

The periodic build is used as this project is complete
and does not require maintenance.

.github/workflows/ci.yml

@@ -1,19 +1,33 @@
 name: CI
 
-on: [ push, pull_request ]
+on:
+  push:
+    branches:
+      - 'v*.*.x'
+    tags:
+      - 'v*'
+  pull_request:
+    branches:
+      - 'v*.*.x'
+  # Do a periodic build (every Monday at 0600) to ensure integrity
+  schedule:
+    - cron: '0 6 * * 1'
 
 jobs:
   build:
     name: Run
     strategy:
       matrix:
-        os: [ ubuntu-latest, macos-latest, windows-latest ]
-        dc: [ dmd-latest, ldc-latest, dmd-2.094.1, ldc-1.24.0 ]
-
+        include:
+            - { os: macos-latest,   dc: ldc-latest }
+            - { os: ubuntu-latest,  dc: dmd-latest }
+            - { os: ubuntu-latest,  dc: ldc-latest }
+            - { os: windows-latest, dc: dmd-latest }
+            - { os: windows-latest, dc: ldc-latest }
     runs-on: ${{ matrix.os }}
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
       with:
         # For codecov
         fetch-depth: 2
@@ -24,9 +38,9 @@ jobs:
     - name: 'Build & Test'
       shell: bash
       run: |
-        dub test --compiler=$DC -b unittest-cov
+        dub test -b unittest-cov
 
     - name: Upload coverage to Codecov
-      uses: codecov/codecov-action@v1
+      uses: codecov/codecov-action@v4
       with:
         flags: unittests