Relaxed semantics for PSK configuration
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This affects users which call the PSK configuration APIs
`mbedtlsl_ssl_conf_psk()` and `mbedtls_ssl_conf_psk_opaque()`
multiple times on the same SSL configuration.

In Mbed TLS 2.x, users would observe later calls overwriting
the effect of earlier calls, with the prevailing PSK being
the one that has been configured last. In Mbed TLS 3.0,
calling `mbedtls_ssl_conf_[opaque_]psk()` multiple times
will return an error, leaving the first PSK intact.

To achieve equivalent functionality when migrating to Mbed TLS 3.0,
users calling `mbedtls_ssl_conf_[opaque_]psk()` multiple times should
remove all but the last call, so that only one call to _either_
`mbedtls_ssl_conf_psk()` _or_ `mbedtls_ssl_conf_psk_opaque()`
remains.