Bump third_party/openthread/repo from `f0b6fce` to `fb7b457` #288

Job	Run time
Test Suites - Darwin (no-ble-asan-clang)	1h 23m 40s
Test Suites - Linux (no-ble-tsan-clang)	1h 25m 41s
Test Suites - Darwin (no-ble-tsan-clang)	1h 19m 50s
REPL Tests - Linux	55m 32s
REPL Tests - Darwin	0s
	5h 4m 43s

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