Build: Remove composer.lock and use composer update in CI

.github/workflows/tests.yml

@@ -31,7 +31,7 @@ jobs:
           if [[ "${{ matrix.php-versions }}" == "7.0" ]]; then
             composer require --dev phpunit/phpunit:"^6.5" --update-with-dependencies
           fi
-          composer install --prefer-dist --no-progress
+          composer update --prefer-dist --no-progress
 
       - name: Run tests
         run: ./vendor/bin/phpunit
@@ -52,7 +52,7 @@ jobs:
           tools: composer:v2, phpcs
 
       - name: Install dependencies
-        run: composer install --prefer-dist --no-progress
+        run: composer update --prefer-dist --no-progress
 
       - name: Run PHPCS
         run: ./vendor/bin/phpcs --standard=WordPress --runtime-set installed_paths vendor/wp-coding-standards/wpcs ./includes ./admin ./wp-plugin-starter-template.php