Bump third_party/mbedtls/repo from `5bc604f` to `2153b1b` #2031

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	2h 0m 51s
Build on Linux (gcc_debug)	1h 3m 40s
Build on Linux (python_lib)	28m 19s
Build on Darwin (clang, python_lib, simulated)	2h 35m 23s
Build on Linux (python lighting-app)	10m 5s
Build on Linux (coverage)	18m 50s
	6h 37m 8s

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