Fallout from removal of `doc_auto_cfg`

[sunipkm]

I am on rustc version 1.90.0, and started getting

derive_more-impl-2.0.1/src/lib.rs:2:29
  |
2 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
  |                             ^^^^^^^^^^^^ feature has been removed
  |
  = note: removed in CURRENT_RUSTC_VERSION; see <https://github.com/rust-lang/rust/pull/138907> for more information
  = note: merged into `doc_cfg`

[tyranron]

@JelteF I think, in light of this, it makes sense to cut out the 2.1.0 release asap, despite the milestone not being fully completed yet, and postpone tasks to the next milestone.