Deprecate mbedtls_cipher_setup_psa()

[mpg]

After #5204 there is no longer any part of the library that calls this function. Since it was mainly created for internal use, let's deprecate it and encourage users to actually migrate to the PSA API instead (see #5208 too).

• Mark the function as deprecated using MBEDTLS_DEPRECATED (see platform_util.h)
• Document the deprecation using \deprecated
• Add a New deprecations ChangeLog entry that recommends use psa_aead_xxx() / psa_cipher_xxx() directly instead.

Note: updating docs/use-psa-crypto.md is out of scope, as it's already done in #5757