Update version badge and clarify testing section headings

[locus313]

This pull request updates the README.md and TESTING.md files to improve documentation clarity, update badges, and enhance security and automation instructions. The changes focus on making deployment and automation instructions clearer, improving security guidance, and refining testing documentation.

Documentation improvements:

• Updated the version badge in the README.md to use the GitHub release API for more accurate and automatic version reporting.
• Added a note clarifying that users.conf must be located in the same directory as sync-ssh-keys.sh, particularly when deploying to custom paths.

Security and automation enhancements:

• Improved the systemd service example by adding comments explaining ProtectHome and ReadWritePaths, clarifying their roles in controlling script access to home directories.
• Enhanced the GitHub Actions workflow example by adding a step to pre-populate known_hosts with the server's host key using ssh-keyscan, removing the less secure StrictHostKeyChecking=no option.

Testing documentation updates:

• Removed a reference to a dry-run test mode in the README.md (since it may not be implemented), and changed "Mock Endpoints" to "External Test Endpoints" in TESTING.md for more accurate terminology. [1] [2]