config.h was split into build_info.h and mbedtls_config.h. build_info.h is intended to be included from C code directly, while mbedtls_config.h is intended to be edited by end users wishing to change the build configuration, and should generally only be included from build_info.h. This is because all the preprocessor logic has been moved into build_info.h, including the handling of the MBEDTLS_CONFIG_FILE macro.
A config file version symbol, MBEDTLS_CONFIG_VERSION was introduced. Defining it to a particular value will ensure that Mbed TLS interprets the config file in a way that's compatible with the config file format used by the Mbed TLS release whose MBEDTLS_VERSION_NUMBER has the same value. The only value supported by Mbed TLS 3.0.0 is 0x03000000.