gha: bump to latest stable PHP version where possible

Author: mfn

.github/workflows/autoformat.yml

@@ -16,7 +16,7 @@ jobs:
         with:
           coverage: none
           extensions: mbstring
-          php-version: 8.1
+          php-version: 8.3
 
       - run: composer install --no-interaction --no-progress --no-suggest
 

.github/workflows/continuous-integration.yml

@@ -16,7 +16,7 @@ jobs:
       - name: "Install PHP with extensions"
         uses: shivammathur/setup-php@v2
         with:
-          php-version: 8.1
+          php-version: 8.3
 
       - name: "Install locked dependencies with composer"
         run: composer install --no-interaction --no-progress
@@ -144,7 +144,7 @@ jobs:
         uses: shivammathur/setup-php@v2
         with:
           coverage: pcov
-          php-version: 8.1
+          php-version: 8.3
 
       - name: "Install locked dependencies with composer"
         run: composer install --no-interaction --no-progress