Replace deprecated microscanner with Trivy security scanner

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This PR updates the multi-stage scanning example to use Trivy instead of the deprecated Aqua Microscanner, providing a modern and actively maintained security scanning solution for Node.js Docker containers.

What Changed

multi-stage-scanning/Dockerfile: Replaced the entire microscanner implementation (Stage 5) with a comprehensive Trivy setup that:

• Downloads and installs Trivy v0.35.0 with multi-architecture support
• Performs filesystem vulnerability scanning with HIGH and CRITICAL severity filtering
• Eliminates the need for API tokens (MICROSCANNER_TOKEN no longer required)
• Uses platform-appropriate installation methods for Alpine Linux

New Documentation: Added multi-stage-scanning/README.md explaining:

• Why microscanner was deprecated and replaced with Trivy
• How to use the updated scanning example
• Build instructions for different stages
• Compatibility notes about the Node.js 10 base image

Enhanced Documentation: Updated ultimate-node-dockerfile/README.md to emphasize Trivy as the modern replacement for microscanner with improved language and links.

Why This Matters

Aqua Microscanner has been deprecated for several years, leaving users with an outdated security scanning tool. Trivy provides:

• Active maintenance and regular updates
• Better vulnerability detection capabilities
• No API token requirements
• Superior performance and reliability
• Multi-platform architecture support

Example Usage

# Build and run security scan
docker build --target audit -t scanning-example:audit .

# Build production image  
docker build --target prod -t scanning-example:prod .

The Trivy implementation scans for HIGH and CRITICAL vulnerabilities and continues the build process even if issues are found, maintaining the same behavior as the original microscanner setup.

Fixes #263.

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