Fix build and update to syn v2

[mleonhard]

This PR fixes #517 . Here's what it does

• Rolls back dependabot changes which broke the build
• Update integration_tests__test9_package_with_git_deps.stdout.snap to match a package version in Cargo.lock
• Migrate from syn v1 to v2 to support new C-style string literals. This fixes the panic: Unrecognized literal: c"".
• Since Rust 1.77 changed the cargo metadata package-ID format (fix(metadata): Stabilize id format as PackageIDSpec  rust-lang/cargo#12914), cargo-geiger built with a Rust 1.77 toolchain always fails with:

  thread 'scan::scan_tests::list_files_used_but_not_scanned_test::case_1' panicked at /Users/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/krates-0.11.0/src/builder.rs:919:36:
  called `Option::unwrap()` on a `None` value
  

  We work around this by specifying the Rust 1.76 toolchain. The proper fix requires upgrading the cargo_metadata crate to v0.18. This will require major changes to cargo-geiger's code.

Problems in this PR:

• Some tests are failing.

[mleonhard]

This PR keeps growing and growing as I fix more tests. I would love to hear suggestions for splitting it up. It would also be great if there was a way to get syn v2 to parse the same way as v1 so expression counts would match.

[pinkforest]

We're re-writing much of the core incl. to break the cargo-reliance so messy PR is ok for now to fix the techdebt np.

Thanks so much for doing this 🚀

[pinkforest]

Pinning causes other problems. We can use rust-version in Cargo.toml for MSRV if we are to bump it.

EDIT: I misread your comment - yeah we need to address the core problem really. Thanks for identifying it.