Bump third_party/openthread/repo from `287dbfa` to `473af53` #436

Job	Run time
Test Suites - Linux (no-ble-no-shell-tsan-clang)	1h 52m 57s
Test Suites - Darwin (no-ble-no-shell-asan-clang)	1h 23m 56s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	1h 56m 43s
REPL Tests - Darwin	0s
REPL Tests - Linux	1h 32m 16s
	6h 45m 52s

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