Bump third_party/ot-br-posix/repo from `bd15f71` to `76bfeef` #403

Job	Run time
REPL Tests - Darwin	0s
REPL Tests - Linux	1h 34m 21s
Test Suites - Linux (no-ble-no-shell-tsan-clang)	2h 0m 55s
Test Suites - Darwin (no-ble-no-shell-asan-clang)	1h 27m 58s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	1h 52m 10s
	6h 55m 24s

Provide feedback