Update and Publish VS 1.21.6 image

[Ralnoc]

This pull request updates the Vintage Story server to version 1.21.6 across the codebase and documentation. The main focus is to ensure consistency in the version used for Docker images, build arguments, and documentation, while also updating image tags to use latest for easier maintenance.

Vintage Story Version Update:

• Updated the VS_VERSION build argument in the Dockerfile from 1.21.5 to 1.21.6 to use the latest server version.
• Changed the version in vintage-story-version.txt from 1.21.5 to 1.21.6 to reflect the new default version.

Documentation and Example Updates:

• Updated all references in README.md to use 1.21.6 instead of 1.21.5, including build commands, environment variable examples, and Docker Compose configuration. [1] [2]
• Changed the Docker image tag in README.md from a specific version (0.0.18) to latest for both standalone and environment variable examples, simplifying future upgrades. [1] [2]

Docker Compose Configuration:

• Removed the explicit VS_VERSION environment variable from docker-compose.yml, as the version is now consistently managed elsewhere.