chore: bump php-actions/composer from 5 to 6 (estahn#30)

Bumps [php-actions/composer](https://github.com/php-actions/composer) from 5 to 6.
- [Release notes](https://github.com/php-actions/composer/releases)
- [Commits](php-actions/composer@v5...v6)

---
updated-dependencies:
- dependency-name: php-actions/composer
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/build.yml

@@ -31,7 +31,7 @@ jobs:
   #         path: /tmp/composer-cache
   #         key: ${{ runner.os }}-${{ hashFiles('**/composer.lock') }}
 
-  #     - uses: php-actions/composer@v5
+  #     - uses: php-actions/composer@v6
 
   #     - uses: php-actions/phpunit@v2
   #       env:
@@ -68,7 +68,7 @@ jobs:
           path: /tmp/composer-cache
           key: ${{ runner.os }}-${{ hashFiles('**/composer.lock') }}
 
-      - uses: php-actions/composer@v5
+      - uses: php-actions/composer@v6
         with:
           php_version: ${{ matrix.php }}