mbedtls: check for feature macros instead of version

mbedtls: check for feature macros instead of version

Drop three interim macros and mbedTLS version checks in favor of feature
macros defined by mbedTLS itself.

These mbedTLS features require mbedTLS 3.6.0/3.6.1 for production.
Earlier versions may have partial/experimental support for them,
disabled by default and (AFAICS) without documented build options
to enable them.

One feature guard already used the native macro instead of the interim
one before this patch. (`MBEDTLS_SSL_SESSION_TICKETS`)

Ref: https://github.com/curl/curl/pull/18254#issuecomment-3182668817

Closes #18271