Update to Rust 1.74 and use new clippy lints table (#8722)

Update to [Rust
1.74](https://blog.rust-lang.org/2023/11/16/Rust-1.74.0.html) and use
the new clippy lints table.

The update itself introduced a new clippy lint about superfluous hashes
in raw strings, which got removed.

I moved our lint config from `rustflags` to the newly stabilized
[workspace.lints](https://doc.rust-lang.org/stable/cargo/reference/workspaces.html#the-lints-table).
One consequence is that we have to `unsafe_code = "warn"` instead of
"forbid" because the latter now actually bans unsafe code:

```
error[E0453]: allow(unsafe_code) incompatible with previous forbid
  --> crates/ruff_source_file/src/newlines.rs:62:17
   |
62 |         #[allow(unsafe_code)]
   |                 ^^^^^^^^^^^ overruled by previous forbid
   |
   = note: `forbid` lint level was set on command line
```

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/ruff_python_index/Cargo.toml

@@ -23,3 +23,6 @@ ruff_text_size = { path = "../ruff_text_size" }
 itertools = { workspace = true }
 
 [dev-dependencies]
+
+[lints]
+workspace = true

crates/ruff_python_index/src/indexer.rs

@@ -238,18 +238,18 @@ mod tests {
 
     #[test]
     fn continuation() {
-        let contents = r#"x = 1"#;
+        let contents = r"x = 1";
         let lxr: Vec<LexResult> = lexer::lex(contents, Mode::Module).collect();
         let indexer = Indexer::from_tokens(&lxr, &Locator::new(contents));
         assert_eq!(indexer.continuation_line_starts(), &[]);
 
-        let contents = r#"
+        let contents = r"
         # Hello, world!
 
 x = 1
 
 y = 2
-        "#
+        "
         .trim();
 
         let lxr: Vec<LexResult> = lexer::lex(contents, Mode::Module).collect();