Update MbedTLS, or switch to OpenSSL

Please specify the following versions when submitting a bug report:
 - HTTP.jl master
 - MbedTLS.jl master

The package `MbedTLS.jl` is based on MbedTLS 2.0 https://github.com/Mbed-TLS/mbedtls which was supported until end of 2024. It might be good to either update `MbedTLS.jl` to using version 3 of MbedTLS, or to switch `HTTP.jl` to using `OpenSSL.jl`.
