Bump third_party/openthread/repo from `b78b71b` to `ac7f751` #1172

Job	Run time
REPL Tests - Linux	33m 35s
Test Suites - Darwin (no-ble-asan-clang)	1h 25m 44s
Test Suites - Darwin (no-ble-tsan-clang)	2h 29m 56s
Test Suites - Linux (no-ble-tsan-clang)	1h 28m 49s
REPL Tests - Darwin	0s
	5h 58m 4s

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