Documentation feature doc_auto_cfg is now removed

[issokuos]

The documentation feature doc_auto_cfg now removed in rust-lang/rust#138907 (comment) breaks the documentation build

 Documenting secrecy v0.10.3
    Checking bon v3.8.2
    Checking secrecy v0.10.3
   Compiling async-trait v0.1.89
error[E0557]: feature has been removed
  --> /Users/xxxx/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/secrecy-0.10.3/src/lib.rs:33:29
   |
33 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
   |                             ^^^^^^^^^^^^ feature has been removed
   |
   = note: removed in 1.92.0; see <https://github.com/rust-lang/rust/pull/138907> for more information
   = note: merged into `doc_cfg`

error: Compilation failed, aborting rustdoc

For more information about this error, try `rustc --explain E0557`.
error: could not document `secrecy`
warning: build failed, waiting for other jobs to finish...

[issokuos]

I can open a PR to replace the feature with doc_cfg if it's okay with you