Remove using namespace SimTK to fix compatibility with abseil 20230802

[traversaro]

🦟 Bug fix

Fixes #3346

Summary

In gazebo/physics/simbody/SimbodyPhysics.cc using namespace SimTK was used, that brought in the global namespace functions such as operator<= that created conflicts with some version of abseil (that is a transitive dependency via protobuf). For clarity and consistency with the rest of the codebase, using namespace SimTK was removed and replaced with explicity use of the namespace SimTK::.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

[scpeters]

macOS CI failed due to graphviz, but I merged with gazebo11 locally, and it compiles, so that's enough for me