`Gaudi::Monitoring::Hub::Entity::operator==` should be const

otherwise we get warnings like this:

../Online/Gaucho/src/components/OnlMonitorSink.cpp:151:67: warning: C++20 says that these are ambiguous, even though the second is reversed:
  151 |                               [&ent]( auto& x ) { return x.second == ent; } );
      |                                                          ~~~~~~~~~^~~~~~
In file included from ../Online/Gaucho/src/components/OnlMonitorSink.h:12,
                 from ../Online/Gaucho/src/components/OnlMonitorSink.cpp:12:
/workspace/build/Gaudi/InstallArea/x86_64_v2-el9-gcc13-opt/include/Gaudi/MonitoringHub.h:111:12: note: candidate 1: 'bool Gaudi::Monitoring::Hub::Entity::operator==(const Gaudi::Monitoring::Hub::Entity&)'
  111 |       bool operator==( Entity const& ent ) { return id() == ent.id(); }
      |            ^~~~~~~~
/workspace/build/Gaudi/InstallArea/x86_64_v2-el9-gcc13-opt/include/Gaudi/MonitoringHub.h:111:12: note: candidate 2: 'bool Gaudi::Monitoring::Hub::Entity::operator==(const Gaudi::Monitoring::Hub::Entity&)' (reversed)
/workspace/build/Gaudi/InstallArea/x86_64_v2-el9-gcc13-opt/include/Gaudi/MonitoringHub.h:111:12: note: try making the operator a 'const' member function