Upgrade Rust toolchain to nightly-2024-06-21 (#3282)

Related change:
    - rust-lang/rust#126623

Resolves #3275

Author: celinval

kani-compiler/src/kani_middle/attributes.rs

@@ -628,7 +628,7 @@ fn parse_modify_values<'a>(
     std::iter::from_fn(move || {
         let tree = iter.next()?;
         let wrong_token_err =
-            || tcx.sess.psess.dcx.span_err(tree.span(), "Unexpected token. Expected identifier.");
+            || tcx.sess.dcx().span_err(tree.span(), "Unexpected token. Expected identifier.");
         let result = match tree {
             TokenTree::Token(token, _) => {
                 if let TokenKind::Ident(id, _) = &token.kind {
@@ -655,7 +655,7 @@ fn parse_modify_values<'a>(
         match iter.next() {
             None | Some(comma_tok!()) => (),
             Some(not_comma) => {
-                tcx.sess.psess.dcx.span_err(
+                tcx.sess.dcx().span_err(
                     not_comma.span(),
                     "Unexpected token, expected end of attribute or comma",
                 );

rust-toolchain.toml

@@ -2,5 +2,5 @@
 # SPDX-License-Identifier: Apache-2.0 OR MIT
 
 [toolchain]
-channel = "nightly-2024-06-18"
+channel = "nightly-2024-06-21"
 components = ["llvm-tools", "rustc-dev", "rust-src", "rustfmt"]