libs: Move favicon URLs to HTTPS

Helps prevent mixed content warnings if accessing docs over HTTPS.

Closes #25459

src/libgraphviz/lib.rs

@@ -279,7 +279,7 @@
 #![crate_type = "rlib"]
 #![crate_type = "dylib"]
 #![doc(html_logo_url = "http://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
-       html_favicon_url = "http://www.rust-lang.org/favicon.ico",
+       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "http://doc.rust-lang.org/nightly/")]
 #![feature(collections)]
 #![feature(into_cow)]