Prepare v4.0: unified database setup and cleanup

[Rafikooo]

Breaking changes for v4.0 release:

• Unified database credentials (sylius:sylius) across MySQL, MariaDB, and PostgreSQL
• Removed deprecated database_version input (use database: "mysql:8.4" format)
• Removed postgresql alias (only postgres supported now)
• Action now sets DATABASE_URL automatically - no need to define it in workflows

Other improvements:

• PostgreSQL CI optimizations (fsync=off, synchronous_commit=off, etc.) for faster test runs
• Fixed required: true on inputs that have defaults
• Merged duplicate PHP setup steps
• Pinned browser-actions/setup-chrome to v2
• Updated node_version default to 22.x

Usage example after v4.0

strategy:
    matrix:
        database: ["mysql:8.4", "postgres:16", "mariadb:10.11"]

steps:
    - uses: actions/checkout@v4

    - name: Build application
      uses: SyliusLabs/BuildTestAppAction@v4
      with:
          database: ${{ matrix.database }}
          php_version: "8.3"
          symfony_version: "^7.4"
          sylius_version: "~2.2.0"
          e2e: "yes"

    - name: Run tests
      run: vendor/bin/behat

No DATABASE_URL in env: needed - the action handles it.