Use importlib.resources once python<3.12 support is dropped

[olliesilvester]

The latest ophyd-async load plan requires a directory (Path or String type), but the importlib.resources only added support to represent a traversable as a directory as of python 3.12 (see https://docs.python.org/3/library/importlib.resources.html). I had to use some old-fashioned Python to get the correct directory path instead, see #770.

Once Python<3.12 is dropped, we should use importlib.resources to get directory locations rather than using
ROOT_DIR = os.path.dirname(os.path.abspath(__file__))

Acceptance Criteria

• Support for python<3.12 is dropped in mx-bluesky
• The ROOT_DIR constant in definitions.py is removed and we use importlib.resources.as_file in its place