Bump third_party/openthread/repo from `2aeb8b8` to `7886083` #407

Job	Run time
REPL Tests - Darwin	0s
REPL Tests - Linux	1h 40m 15s
Test Suites - Darwin (no-ble-no-shell-asan-clang)	1h 18m 13s
Test Suites - Linux (no-ble-no-shell-tsan-clang)	2h 5m 58s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	2h 2m 21s
	7h 6m 47s

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