Bump third_party/ot-br-posix/repo from `bd15f71` to `f2a7208` #342

Job	Run time
REPL Tests - Darwin	0s
REPL Tests - Linux	1h 40m 44s
Test Suites - Darwin (no-ble-asan-clang)	1h 25m 35s
Test Suites - Linux (no-ble-tsan-clang)	1h 53m 57s
Test Suites - Darwin (no-ble-tsan-clang)	1h 48m 5s
	6h 48m 21s

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