Bump third_party/ot-br-posix/repo from `8299da3` to `4e75260` #977

Job	Run time
REPL Tests - Darwin	0s
REPL Tests - Linux	35m 24s
Test Suites - Darwin (no-ble-asan-clang)	1h 30m 30s
Test Suites - Darwin (no-ble-tsan-clang)	2h 4m 30s
Test Suites - Linux (no-ble-tsan-clang)	1h 47m 17s
	5h 57m 41s

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