Fix remaining references to policy gradient. #9273

Job	Run time
benchmark (e10_drive)	3m 13s
examples-rl (e10_drive)	5m 19s
benchmark (e11_platoon)	5m 4s
examples-rl (e11_platoon)	5m 16s
base-tests (./cli)	2m 48s
base-tests (./envision)	3m 18s
base-tests (./smarts/core --nb-exec-timeout 65536 --ignore=./smarts/core/tests/test_notebook.py)	4m 26s
base-tests (./smarts/env --ignore=./smarts/env/tests/test_rllib_hiway_env.py)	4m 50s
base-tests (./smarts/env/tests/test_rllib_hiway_env.py)	3m 7s
base-tests (./smarts/sstudio)	2m 49s
base-tests (./smarts/ray)	3m 8s
base-tests (./examples/tests/test_examples.py)	9m 8s
	52m 26s

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