Bump Rust to version 1.77

[wprzytula]

When trying to build rowan we get this error

error: package `rowan v0.15.16` cannot be built because
it requires rustc 1.77.0 or newer, while the currently active rustc
version is 1.75.0

So let's upgrade to 1.77 to allow us to build rowan.

[wprzytula]

@jrvanwhy

[lschuermann]

LGTM, but seems like some syntax changed around thread_local!:

 error: initializer for `thread_local` value can be made `const`
  --> unittest/src/kernel_data.rs:31:76
   |
31 | thread_local!(pub(crate) static KERNEL_DATA: RefCell<Option<KernelData>> = RefCell::new(None));
   |                                                                            ^^^^^^^^^^^^^^^^^^ help: replace with: `const { RefCell::new(None) }`

[wprzytula]

Now CI (clippy) complains about optimisations possible with the new toolchain. Nothing to be done with this PR.

[lschuermann]

Now CI (clippy) complains about optimisations possible with the new toolchain. Nothing to be done with this PR.

Typically we do address those in tandem with version updates. This PR should pass CI on its own.