Bump third_party/ot-br-posix/repo from `bd15f71` to `76bfeef` #402

Job	Run time
Build framework (macos-release, -sdk macosx -configuration Release)	17m 26s
Build framework (ios-release, -sdk iphoneos -configuration Release)	11m 53s
Build framework (tvos-debug, -sdk appletvos -configuration Debug)	11m 49s
Build framework (watchos-debug, -sdk watchos -configuration Debug)	15m 44s
Run framework tests (asan, -enableAddressSanitizer YES -enableUndefinedBehaviorSanitizer YES)	29m 50s
Run framework tests (tsan, -enableThreadSanitizer YES)	38m 9s
Run framework tests (leaks, ENABLE_LEAK_DETECTION=1)	36m 25s
	2h 41m 16s

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