Update VPN troubleshoot script for 2025 infrastructure

[aprets]

Summary

Updates the TryHackMe VPN troubleshooting script for the new VPN infrastructure rolled out in 2025.

Thanks to @LouisTHM0 for the initial changes.

Changes

• IP Range: Updated detection from 10.X.X.X to 192.168.X.X
• VPN Server: Changed connectivity target from 10.10.10.10 to 192.168.128.1
• OpenVPN 2.5 Support: Added detection for inline <auth-user-pass> incompatibility with guidance
• MTU Detection: Improved with retry logic to prevent false positives from packet loss
• Version Flag: Added --version / -v support
• URLs: Updated all references to /manage-account/access
• Bug Fixes: Fixed temp file cleanup, sed newline issues
• Documentation: Updated README.md and README_RU.md
• Testing: Added test suite (tests/test-thm-troubleshoot.sh)

Testing

All 9 tests pass on Ubuntu 22.04:

• Version flag
• OpenVPN installation
• Inline auth detection
• Successful connection
• Correct IP range detection
• MTU check (normal)
• MTU issue detection (constrained network)
• MTU fix application
• MTU config file verification