[CoreML] Use CoreMLTools Main Branch

[YifanShenSZ]

When we were prototyping the integration between ExecuTorch and CoreML, we were using the prototype branch executorch_integration

...
echo "${green}ExecuTorch: Checking out 'executorch_integration' branch in coremltools repo."
git checkout executorch_integration
...

in backends/apple/coreml/scripts/install_requirements.sh

Now that we have a more mature integration and have all necessary functionalities in main branch, it's time to use main branch now

[YifanShenSZ]

Fixed by PR 1666