Bump third_party/openthread/repo from `66eaeec` to `a8afffb` #1133

Job	Run time
REPL Tests - Darwin	0s
REPL Tests - Linux	27m 45s
Test Suites - Darwin (no-ble-asan-clang)	1h 26m 33s
Test Suites - Darwin (no-ble-tsan-clang)	1h 44m 41s
Test Suites - Linux (no-ble-tsan-clang)	1h 18m 34s
	4h 57m 33s

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