Update MbedTLS, or switch to OpenSSL

[eschnett]

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.