Update CI matrix and actions

Test PHP 8.2 and 8.3, run on pull requests, and stop using the retired
actions/checkout@v2 and ramsey/composer-install@v1.

The lowest-dependency jobs are limited to PHP 7.3-8.0: Slim 3.0, Pimple
3.0 and vfsStream 1.6.8 predate PHP 8.1 and fatal on load there. Raising
those dev bounds is not an option either, because the first Slim 3 release
that supports PHP 8.1 (3.13.0) requires PHP 8.1 and would break the 7.3
and 7.4 jobs. Every supported PHP version stays covered by the highest
jobs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: snapshotpl

.github/workflows/tests.yml

@@ -1,25 +1,27 @@
 name: CI
 on:
     - push
+    - pull_request
 jobs:
     phpstan:
         runs-on: ubuntu-latest
         steps:
             -   name: Checkout
-                uses: actions/checkout@v2
+                uses: actions/checkout@v4
 
             -   name: Setup PHP, with composer and extensions
                 uses: shivammathur/setup-php@v2
                 with:
                     php-version: 7.3
 
             -   name: Install dependencies with Composer
-                uses: ramsey/composer-install@v1
+                uses: ramsey/composer-install@v3
 
             -   name: Run phpstan
                 run: vendor/bin/phpstan analyse --level=6 src/
     tests:
         strategy:
+            fail-fast: false
             matrix:
                 dependencies:
                     - highest
@@ -29,18 +31,31 @@ jobs:
                     - 7.4
                     - 8.0
                     - 8.1
+                    - 8.2
+                    - 8.3
+                exclude:
+                    # Lowest bounds of our dependencies were released before PHP 8.1
+                    # and do not carry #[\ReturnTypeWillChange], so they fatal on load.
+                    # This is a limitation of those old releases, not of this library:
+                    # every supported PHP version is covered by the highest jobs.
+                    -   dependencies: lowest
+                        php-versions: 8.1
+                    -   dependencies: lowest
+                        php-versions: 8.2
+                    -   dependencies: lowest
+                        php-versions: 8.3
         runs-on: ubuntu-latest
         steps:
             -   name: Checkout
-                uses: actions/checkout@v2
+                uses: actions/checkout@v4
 
             -   name: Setup PHP, with composer and extensions
                 uses: shivammathur/setup-php@v2
                 with:
                     php-version: ${{ matrix.php-versions }}
 
             -   name: Install dependencies with Composer
-                uses: ramsey/composer-install@v1
+                uses: ramsey/composer-install@v3
                 with:
                     dependency-versions: ${{ matrix.dependencies }}