Update rust to 1.86.0 (#11431)

We keep the practice of keeping the compiler up to date, pointing to the
latest release. This is done by many other projects in the Rust
ecosystem as well.

[Announcement blog
post](https://blog.rust-lang.org/2025/04/03/Rust-1.86.0.html).

Prior update was in #10914.

storage_scrubber/src/find_large_objects.rs

@@ -18,7 +18,7 @@ enum LargeObjectKind {
 
 impl LargeObjectKind {
     fn from_key(key: &str) -> Self {
-        let fname = key.split('/').last().unwrap();
+        let fname = key.split('/').next_back().unwrap();
 
         let Ok((layer_name, _generation)) = parse_layer_object_name(fname) else {
             return LargeObjectKind::Other;