6th generation limits

The performance-monitoring tool and more specifically https://github.com/imperva/mx-toolbox/blob/3b47495ca2e89056ab84678fbfbfecfe6b27d0d7/performance-monitoring/get_gateway_stats.py#L608-L641 does not seem to contain the 6th gen (X*520 ) device limits

Start by reading performance-monitoring/get_gateway_stats.py around lines 608-641 and inspect how device limits are represented and used. Identify the missing 6th-generation X*520 limits, update that limits handling, and verify the performance-monitoring tool includes them in its gateway statistics behavior.

Written by the indexing model from the issue text.