mbedtls: fix private member designations for v3.1.0

mbedtls: fix private member designations for v3.1.0

"As a last resort, you can access the field foo of a structure bar by
writing bar.MBEDTLS_PRIVATE(foo). Note that you do so at your own risk,
since such code is likely to break in a future minor version of Mbed
TLS." -
https://github.com/ARMmbed/mbedtls/blob/f2d1199edc5834df4297f247f213e614f7782d1d/docs/3.0-migration-guide.md

That future minor version is v3.1.0. I set the >= to == for the version
checks because v3.1.0 is a release, and I am not sure when the private
designation was reverted after v3.0.0.

Closes #8214