fix CI: bump PHP to 8.3, remove composer.lock from tracking

Lock file had L13 deps (PHP 8.3+) committed, breaking the lint
workflow on PHP 8.2. Libraries shouldn't track lock files anyway.
Also bumped checkout action to v6.

Author: pdphilip

.github/workflows/lint.yml

@@ -5,7 +5,7 @@ on:
   pull_request:
 
 env:
-  PHP_VERSION: "8.2"
+  PHP_VERSION: "8.3"
 
 jobs:
   phplint:
@@ -19,7 +19,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v4"
+        uses: "actions/checkout@v6"
 
       - name: "Install PHP"
         uses: "shivammathur/setup-php@v2"

.github/workflows/phpstan.yml

@@ -16,12 +16,12 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 5
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
-          php-version: '8.2'
+          php-version: '8.3'
           coverage: none
 
       - name: Install composer dependencies