`doc_auto_cfg` removed

[tarcieri]

https://docs.rs/crate/elliptic-curve/0.14.0-rc.15/builds/2547002

[INFO] [stderr] error[E0557]: feature has been removed
[INFO] [stderr]  --> src/lib.rs:2:29
[INFO] [stderr]   |
[INFO] [stderr] 2 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
[INFO] [stderr]   |                             ^^^^^^^^^^^^ feature has been removed
[INFO] [stderr]   |
[INFO] [stderr]   = note: removed in 1.58.0; see <https://github.com/rust-lang/rust/pull/138907> for more information
[INFO] [stderr]   = note: merged into `doc_cfg`

This will break all of our docsrs builds in their current form.

I think we can migrate (back) to doc_cfg now? Haven't tested it out

Yes, doc_cfg has the same behavior as the old doc_auto_cfg now.