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

<!--
Thank you for contributing to Ruff! To help us out with reviewing,
please consider the following:

- Does this pull request include a summary of the change? (See below.)
- Does this pull request include a descriptive title?
- Does this pull request include references to any relevant issues?
-->

## 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_dev/src/generate_docs.rs

@@ -2,6 +2,7 @@
 #![allow(clippy::print_stdout, clippy::print_stderr)]
 
 use std::collections::HashSet;
+use std::fmt::Write as _;
 use std::fs;
 use std::path::PathBuf;
 
@@ -29,8 +30,7 @@ pub(crate) fn main(args: &Args) -> Result<()> {
         if let Some(explanation) = rule.explanation() {
             let mut output = String::new();
 
-            output.push_str(&format!("# {} ({})", rule.as_ref(), rule.noqa_code()));
-            output.push('\n');
+            let _ = writeln!(&mut output, "# {} ({})", rule.as_ref(), rule.noqa_code());
 
             let (linter, _) = Linter::parse_code(&rule.noqa_code().to_string()).unwrap();
             if linter.url().is_some() {
@@ -49,11 +49,12 @@ pub(crate) fn main(args: &Args) -> Result<()> {
                     common_prefix.to_lowercase()
                 );
 
-                output.push_str(&format!(
+                let _ = write!(
+                    output,
                     "Derived from the **[{}](../rules.md#{})** linter.",
                     linter.name(),
-                    anchor
-                ));
+                    anchor,
+                );
                 output.push('\n');
                 output.push('\n');
             }
@@ -155,8 +156,8 @@ fn process_documentation(documentation: &str, out: &mut String, rule_name: &str)
                 }
 
                 let anchor = option.replace('.', "_");
-                out.push_str(&format!("- [`{option}`][{option}]\n"));
-                after.push_str(&format!("[{option}]: ../settings.md#{anchor}\n"));
+                let _ = writeln!(out, "- [`{option}`][{option}]");
+                let _ = writeln!(&mut after, "[{option}]: ../settings.md#{anchor}");
                 referenced_options.insert(option);
 
                 continue;
@@ -171,7 +172,7 @@ fn process_documentation(documentation: &str, out: &mut String, rule_name: &str)
         if let Some(OptionEntry::Field(field)) = Options::metadata().find(option) {
             if referenced_options.insert(option) {
                 let anchor = option.replace('.', "_");
-                after.push_str(&format!("[{option}]: ../settings.md#{anchor}\n"));
+                let _ = writeln!(&mut after, "[{option}]: ../settings.md#{anchor}");
             }
             if field.deprecated.is_some() {
                 eprintln!("Rule {rule_name} references deprecated option {option}.");