rustdoc: stabilise --default-theme

As discussed in #77213, this seems like it has bedded in and can be
safely and usefully made stable.

(rustdoc already has other stable options that interact quite
intimately with the rustdoc-supplied CSS, and also an option for
supplying entirely different CSS, so exposing the theme names this way
seems a very minor step.)

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

src/librustdoc/lib.rs

@@ -269,7 +269,7 @@ fn opts() -> Vec<RustcOptGroup> {
                 "sort modules by where they appear in the program, rather than alphabetically",
             )
         }),
-        unstable("default-theme", |o| {
+        stable("default-theme", |o| {
             o.optopt(
                 "",
                 "default-theme",