Various fixes too make the CI green

[hkratz]

Split into semantically separate commits for easier reviewing.

compiler warnings:

• Fix unexpected cfg condition name: ... warnings introduced in Rust 1.80 (see https://blog.rust-lang.org/2024/05/06/check-cfg.html)
• io_slice_advance feature is now stable

clippy lints:

• missing_const_for_thread_local lint was renamed to missing_const_for_thread_local, but disabling it is no longer necessary since thread_local_initializer_can_be_made_const should respect MSRV rust-clippy#12404 has been fixed

clippy doc lints:

• fixes for "doc list item without indentation" lint
• ignore incorrect "first doc comment paragraph is too long" warning (see too_long_first_doc_paragraph only considers markdown source not the rendered output rust-clippy#13315)
• allow long first paragraphs in select... fn doc comments

[taiki-e]

Thanks! LGTM aside from a nit.