Simplify download and/or TLS backends

[rami3l]

Part of #1611.

Motivation

The direct motivation of this change is to simplify Rustup's building process in order to make Rustup more cross-build friendly and prevent surprise stable-only CI build errors like the ones we've encountered while bumping OpenSSL to v3, i.e. #3478, #3668, etc.

Status quo

• The default currently is to use reqwest with native-tls (OpenSSL on Linux, native stacks on macOS and Windows)
• If RUSTUP_USE_CURL is set, we use the curl backend, which defaults to openssl
• If RUSTUP_USE_RUSTLS is set, we use reqwest with rustls

#3788 (comment)

Goal

The eventual goal would be removing RUSTUP_USE_CURL and/or RUSTUP_USE_RUSTLS, leaving a single stack "in favor of maximally-Rust solutions", while "making sure we minimize the risk of getting users stuck".

#3788 (comment)

Plan

• Unfreeze OpenSSL so we can keep it updated again
• Include tracing support with tracing-subscriber by default
  • Make sure we don't constrain the trace!() level statically
  • Maybe remove opentelemetry stuff
• Switch the default from reqwest/native-tls to reqwest/rustls, keeping the curl/openssl fallback for now
• Collect user feedback
• Remove curl/openssl and reqwest/native-tls, leaving only reqwest/rustls

#3788 (comment)

Tasks

Beta Give feedback

1. chore(deps): unpin openssl version #3793
   dependencies +1
2. chore(dist/features): ship tracing and friends by default #3803
   +0
3. feat(download-backend)!: make reqwest/rustls the new default #3798
   +0
4. CfT: Test out Rustup's reqwest backend with rustls #3806
   0 of 1
   help wanted meta +2
5. feat(download): reflect the download/TLS backends in the user agent #3815
   +0
6. refactor(download): remove curl backend #3788
   +0

[djc]

Also, many benefits from rustls just being more secure by default than OpenSSL.