Bump third_party/openthread/repo from `287dbfa` to `473af53` #2412

Job	Run time
Test Suites - Darwin (no-ble-no-shell-asan-clang)	1h 30m 56s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	2h 11m 48s
Test Suites - Linux (no-ble-no-shell-tsan-clang)	1h 44m 23s
REPL Tests - Darwin	0s
REPL Tests - Linux	1h 41m 37s
	7h 8m 44s

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