Bump third_party/ot-br-posix/repo from `bd15f71` to `7fd5bd1` #431

Job	Run time
REPL Tests - Darwin	0s
REPL Tests - Linux	1h 31m 40s
Test Suites - Linux (no-ble-no-shell-tsan-clang)	1h 57m 0s
Test Suites - Darwin (no-ble-no-shell-asan-clang)	1h 38m 9s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	1h 38m 11s
	6h 45m 0s

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