Rename docs_rs config condition to docsrs

[dannycjones]

Description of change

This change is made to avoid config condition errors now that check config is run by default in Rust 1.80+.

Without this change, compilation will fail like below:

error: unexpected `cfg` condition name: `docs_rs`
    --> mountpoint-s3-client/src/s3_crt_client.rs:1162:16
     |
1162 | #[cfg_attr(not(docs_rs), async_trait)]
     |                ^^^^^^^ help: there is a config with a similar name: `docsrs`
     |
     = help: consider using a Cargo feature instead
     = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
              [lints.rust]
              unexpected_cfgs = { level = "warn", check-cfg = ['cfg(docs_rs)'] }
     = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(docs_rs)");` to the top of the `build.rs`
     = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration

Rather than specifying the docs_rs config condition in our Cargo.toml, we can use the one already existing and documented by docs.rs: https://docs.rs/about/builds#detecting-docsrs

To recognize Docs.rs from your Rust code, you can test for the docsrs cfg, e.g.:

#[cfg(docsrs)]
mod documentation;

This change removes the setting of docs_rs cfg condition and updates all conditions to use the existing condition set by docs.rs.

Checked using the following command that the docs aren't a mess by looking at mountpoint_s3_client::object_client::PutObjectRequest:

RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --no-deps

Fixes CI issue blocking PR #949.

Relevant issues: N/A

Does this change impact existing behavior?

No.

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