SimpleTrackedVehiclePlugin: fix for boost 1.74

[scpeters]

Our macOS builds are failing since homebrew updated to boost 1.74 in Homebrew/homebrew-core#62549:

• https://build.osrfoundation.org/job/gazebo-ci-gazebo11-homebrew-amd64/39/

plugins/SimpleTrackedVehiclePlugin.cc:35:7: error: redefinition of 'hash<boost::shared_ptr<T> >'
class hash<boost::shared_ptr<T>> {
      ^~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/boost/smart_ptr/shared_ptr.hpp:1174:26: note: previous definition is here
template<class T> struct hash< ::boost::shared_ptr<T> >
                         ^

It looks like shared_ptr.hpp has changed in 1.74.0. I've disabled some code in SimpleTrackedVehiclePlugin for boost 1.74 or greater, which seems to fix this.

[scpeters]

the build is fixed:

https://build.osrfoundation.org/job/gazebo-ci-pr_any-homebrew-amd64/2192/

[ahcorde]

@osrf-jenkins retest this please

[j-rivero]

It looks like shared_ptr.hpp has changed in 1.74.0.

This seems correct. Changes in 1.74.0

    Added owner_equals to shared_ptr, weak_ptr, local_shared_ptr
    Added owner_hash_value to shared_ptr, weak_ptr
    Added owner_equal_to, owner_hash
    Added std::hash specializations for shared_ptr, local_shared_ptr
    Added boost::hash support to, and std::hash, std::equal_to specializations for, weak_ptr

[scpeters]

none of the test failures are related to this PR; merging