Proxy 0.6.0 released

• Improvement: Update to Go 1.26, which comes with a new garbage collector implementation that is expected to reduce garbage collection overhead and thus improve performance.
• Bugfix: Prevent a possible crash when the Tideways backend returns a server error.
• Change: Add support for response_time_target_as_error project setting
• Change: Update dependencies and rebuild with the current Golang version.