CI: run Phing 3 via setup-php instead of the abandoned phing-github-action

phingofficial/phing-github-action@main (unmaintained since 2020) builds its own
Docker image from composer:1.10, whose Composer 1.x phar now fatals on PHP 8.4
before any UNA code runs, so every UNA CI run has failed for weeks.

Install PHP 8.3 with intl and zip plus a pinned Phing 3.1.2 through
shivammathur/setup-php and run the package target directly. Verified locally:
`phing package -Dpackage_file=UNA.zip` builds the archive with Phing 3.1.2
(intl is required by its tstamp task). Also bump checkout/cache to v4.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -15,14 +15,21 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
+
+    - name: Setup PHP
+      uses: shivammathur/setup-php@v2
+      with:
+        php-version: '8.3'
+        extensions: intl, zip
+        tools: composer, phing:3.1.2
 
     - name: Validate composer.json and composer.lock
       run: composer validate
 
     - name: Cache Composer packages
       id: composer-cache
-      uses: actions/cache@v3
+      uses: actions/cache@v4
       with:
         path: plugins
         key: ${{ runner.os }}-php-${{ hashFiles('**/composer.lock') }}
@@ -32,10 +39,7 @@ jobs:
       run: composer install --prefer-dist --no-progress
 
     - name: Package zip
-      uses: phingofficial/phing-github-action@main
-      with:
-        targets: package
-        user-properties: package_file=UNA.zip
+      run: phing package -Dpackage_file=UNA.zip
 
     # Add a test script to composer.json, for instance: "test": "vendor/bin/phpunit"
     # Docs: https://getcomposer.org/doc/articles/scripts.md