Bump third_party/ot-br-posix/repo from `bd15f71` to `b574b6c` #6069

Job	Run time
Test Suites - Linux (no-ble-no-shell-tsan-clang)	2h 0m 19s
REPL Tests - Darwin	0s
REPL Tests - Linux	1h 35m 11s
Test Suites - Darwin (no-ble-no-shell-asan-clang)	1h 23m 41s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	2h 9m 28s
	7h 8m 39s

Provide feedback