Fix Python 3.14 package resource loading

[bjornrun]

Summary

• replace the pkg_resources fallback in projectairsim.utils.load_text_resource with importlib.resources.files()
• add the maintained importlib-resources backport dependency for Python <3.9

Verification

• python3 -m compileall -q client/python/projectairsim/src/projectairsim/utils.py
• isolated Python 3.14 resource-load check for schema/robot_config_schema.jsonc, confirming pkg_resources is not imported
• git diff --check -- client/python/projectairsim/src/projectairsim/utils.py client/python/projectairsim/pyproject.toml

Full package import was not run because this local Python 3.14 environment does not have the package runtime dependencies installed (msgpack fails first).