Tracking issue for release notes of #129687: Implement RFC3137 trim-paths sysroot changes - take 2

[rustbot]

This issue tracks the release notes text for #129687.

Steps

• Proposed text is drafted by PR author (or team) making the noteworthy change.
• Issue is nominated for release team review of clarity for wider audience.
• Release team includes text in release notes/blog posts.

Release notes text

The responsible team for the underlying change should edit this section to replace the automatically generated link with a succinct description of what changed, drawing upon text proposed by the author (either in discussion or through direct editing).

# Compatibility notes
- Until now, paths into the sysroot were always prefixed with `/rustc/$hash` in diagnostics, codegen, backtrace, e.g.

    ```
    thread 'main' panicked at 'hello world', map-panic.rs:2:50
    stack backtrace:
       0: std::panicking::begin_panic
                 at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:616:12
       1: map_panic::main::{{closure}}
                 at ./map-panic.rs:2:50
       2: core::option::Option<T>::map
                 at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/option.rs:929:29
       3: map_panic::main
                 at ./map-panic.rs:2:30
       4: core::ops::function::FnOnce::call_once
                 at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/ops/function.rs:248:5
    note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
    ```
    
    [RFC 3127 said](https://rust-lang.github.io/rfcs/3127-trim-paths.html#changing-handling-of-sysroot-path-in-rustc)

    > We want to change this behaviour such that, when `rust-src` source files can be discovered, the virtual path is discarded and therefore the local path will be embedded, unless there is a `--remap-path-prefix` that causes this local path to be remapped in the usual way.

    [#129687](https://github.com/rust-lang/rust/pull/129687) implements this behaviour, when `rust-src` is present at compile time, `rustc` replaces `/rustc/$hash` with a real path into the local `rust-src` component with best effort.
    
    To sanitize this, users must explicitly supply `--remap-path-prefix=<path to rust-src>=foo` or not have the `rust-src` component installed.

Tip

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Release blog section

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Otherwise leave it empty.

cc @Urgau, @jieyouxu -- origin issue/PR authors and assignees for starting to draft text