Rust: Say that we support rust stable in GettingStartedRust.md

In 24765, we said that we supported rust stable in CodingStandardsRust.md.
But we left GettingStartedRust.md saying that we support rust nightly.

Closes 27160.

doc/HACKING/GettingStartedRust.md

@@ -119,9 +119,10 @@ the module calls.  Modules which call fewer other modules are better targets.
 
 Strive to change the C API as little as possible.
 
-We are currently targetting Rust nightly, *for now*. We expect this to change
-moving forward, as we understand more about which nightly features we need. It
-is on our TODO list to try to cultivate good standing with various distro
+We are currently targetting Rust stable. (See CodingStandardsRust.md for more
+details.)
+
+It is on our TODO list to try to cultivate good standing with various distro
 maintainers of `rustc` and `cargo`, in order to ensure that whatever version we
 solidify on is readily available.