Bump third_party/mbedtls/repo from `5bc604f` to `9870921` #2371

Job	Run time
REPL Tests - Linux	1h 33m 56s
Test Suites - Darwin (no-ble-no-shell-asan-clang)	1h 21m 28s
Test Suites - Linux (no-ble-no-shell-tsan-clang)	1h 56m 48s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	1h 46m 17s
REPL Tests - Darwin	0s
	6h 38m 29s

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