Replaced set_difference to copy_if because it is used on unordered_set

[achim-k]

Public-Facing Changes

• Correct calculation of removed topics and services while updating connection graph.

Description

Original commit: foxglove/ws-protocol#437

Now in websocket_server.hpp we have:

https://github.com/foxglove/ws-protocol/blob/5cdda39d9ed4e8287a5d27406f5e4f368bd0e1fc/cpp/foxglove-websocket/include/foxglove/websocket/websocket_server.hpp#L1158-L1159

And to find difference between them function set_difference is used:

https://github.com/foxglove/ws-protocol/blob/5cdda39d9ed4e8287a5d27406f5e4f368bd0e1fc/cpp/foxglove-websocket/include/foxglove/websocket/websocket_server.hpp#L1203-L1206

According to documentation of std::set_difference it can be used only for sorted ranges (which is not a case when we use unorderd_set).

To have O(1) complexity for insert and search operations on unordered_set, I suggest to use std::copy_if function to find difference between two unordered sets.