Do not use NotBefore=0 in certs validity periods #2755

Job	Run time
build_and_test (rustcrypto, alloc)	6m 56s
build_and_test (mbedtls, os)	11m 25s
build_and_test (rustcrypto, os)	25m 23s
build_and_test (rustcrypto)	9m 26s
build_and_test (openssl, os)	6m 48s
build_and_test (mbedtls)	8m 9s
build_and_test (openssl)	7m 16s
build_and_test (mbedtls, alloc)	7m 42s
build_and_test (openssl, alloc)	7m 55s
	1h 31m 0s

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