chore(proxy/http): replace hyper::Body with BoxBody

[cratelyn]

UpgradeResponseBody currently wraps a hyper::Body. this type is removed in hyper 1.0.

this commit replaces this with a generic B-typed body.

see #3479, which performs the same change in linkerd-http-upgrade.

see linkerd/linkerd2#8733 for more information on upgrading to hyper 1.0.

[cratelyn]

this is a somewhat surprising ci error to see! it doesn't seem obviously related to these changes. i opened #3481 to confirm that ci is not currently broken.

ci error

error: failed to run custom build command for `linkerd-app-core v0.1.0 (/__w/linkerd2-proxy/linkerd2-proxy/linkerd/app/core)`

Caused by:
  process didn't exit successfully: `/__w/linkerd2-proxy/linkerd2-proxy/target/debug/build/linkerd-app-core-845976ab62a36e12/build-script-build` (exit status: 101)
  --- stdout
  cargo:rustc-env=GIT_SHA=0262025

  cargo:rustc-env=LINKERD2_PROXY_BUILD_DATE=2024-12-20T00:41:25Z


  --- stderr
  thread 'main' panicked at linkerd/app/core/build.rs:18:17:
  LINKERD2_PROXY_VERSION must be semver
  stack backtrace:
     0: rust_begin_unwind
               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/panicking.rs:665:5
     1: core::panicking::panic_fmt
               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/core/src/panicking.rs:74:14
     2: build_script_build::version
               at ./build.rs:18:17
     3: build_script_build::main
               at ./build.rs:43:59
     4: core::ops::function::FnOnce::call_once
               at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/core/src/ops/function.rs:250:5
  note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
warning: build failed, waiting for other jobs to finish...
Error: build failed
error: Recipe `clippy` failed on line 92 with exit code 1
error: Recipe `clippy` failed on line 92 with exit code 1
Error: Process completed with exit code 1.

[cratelyn]

#3482 tweaks the build script in question so that we'll get a better error in this situation in the future.