Bump third_party/openthread/repo from `b78b71b` to `ac7f751` #1171

Job	Run time
REPL Tests - Linux	33m 18s
Test Suites - Linux (no-ble-tsan-clang)	1h 42m 7s
Test Suites - Darwin (no-ble-asan-clang)	1h 19m 19s
Test Suites - Darwin (no-ble-tsan-clang)	2h 2m 35s
REPL Tests - Darwin	0s
	5h 37m 19s

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