Upgrade to Rust 1.86 and bump MSRV to 1.84 (#17171)

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## Summary

I decided to disable the new
[`needless_continue`](https://rust-lang.github.io/rust-clippy/master/index.html#needless_continue)
rule because I often found the explicit `continue` more readable over an
empty block or having to invert the condition of an other branch.


## Test Plan

`cargo test`

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ruff_python_formatter/src/string/normalize.rs

@@ -679,7 +679,7 @@ pub(crate) fn normalize_string(
                                 output.push_str(&input[last_index..escape_start_offset]);
                                 output.push_str(normalised);
                                 last_index = escape_start_offset + normalised.len();
-                            };
+                            }
 
                             // Move the `chars` iterator passed the escape sequence.
                             // Simply reassigning `chars` doesn't work because the indices` would