Create branch for foxy

[jonbinney]

The change to how parameters are declared in ros2/rclcpp#1522 means there isn't a good way to declare a dynamic parameter that works on both foxy and galactic/rolling. Options I've considered:

1. Using SFINAE could to check for the existence of ParameterDescriptor::dynamic_typing. This type of code seems too unreadable.
2. In CMakeLists.txt, set a compiler definition for the version of rclcpp. Then use that to determine how to create a dynamic variable. This works within this package, but since the code is in a header file, anyone other package who uses it would need to define the same variable. Seems like it could cause problems downstream.

Since we should only really have bugfixes on foxy at this point, I think backporting from the ros2 branch shouldn't be too much of a burden. I'll create the new foxy branch and do a release from it (I assume that jenkins is using the releases to figure out what branch to test with which rosdistro....?)

[jonbinney]

Branch created and release made from the new branch: ros/rosdistro#30191

I'll close this issue once the release gets merged.