[BUG] Build fails on latest nightly

[krojew]

Describe the bug
Build fails on latest nightly:

error[E0557]: feature has been removed
   --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/elasticsearch-7.12.0-alpha.1/src/lib.rs:352:39
    |
352 | #![cfg_attr(RUSTC_IS_NIGHTLY, feature(external_doc))]
    |                                       ^^^^^^^^^^^^ feature has been removed
    |
    = note: use #[doc = include_str!("filename")] instead, which handles macro invocations

To Reproduce
Steps to reproduce the behavior:

1. Use rustc 1.54.0-nightly (35fff69d0 2021-06-06)
2. Try to build.

Expected behavior
Success.

Environment (please complete the following information):

• OS: Ubuntu
• rustc version: x86_64-unknown-linux-gnu 1.54.0-nightly (35fff69d0 2021-06-06)