CI: Fix the CI (remove Skeptic etc.) for v0.11.x

[tatsuya6502]

• Update the doc tests on the README.md by replacing Skeptic with a regular cargo test command.
• Rename a CI job Skeptic to Trybuild.
• Rename .cargo/config to .cargo/config.toml.
• Fix the CI for nightly + minimal crate versions by pinning ahash to v0.8.7.
• Fix beta Clippy warnings.
  • clippy 0.1.77 (f2048098a1c 2024-02-09)
• Fix beta compiler warnings.
  • rustc 1.77.0-beta.2 (f2048098a 2024-02-09)
• Replace actions-rs/clippy-check with a regular GHA run command.
• Disable the fail-fast flag for the Lints and Trybuild jobs.