chore: prepare fastpool for archival

[tisonkun]

Summary

• mark the entire Fastpool crate deprecated as of 1.1.2 and direct users to asyncband::pool
• add a prominent README warning and concrete migration notes for the API differences
• keep Fastpool's implementation, AsyncBand 0.6.7 dependency, and Rust 1.85 MSRV unchanged
• bump the crate from 1.1.1 to 1.1.2 so the deprecation can be published as the final planned release

The crate-level deprecation is rendered on the rustdoc landing page, while crates.io will show the deprecated package description and README warning. After this PR merges, publish Fastpool 1.1.2 before archiving the GitHub repository.

Testing

• cargo +nightly fmt --all -- --check
• cargo x build --locked
• cargo x test --no-capture
• cargo x lint
• cargo publish -p fastpool --dry-run --allow-dirty
• cargo doc -p fastpool --no-deps