refactor!: remove cloud feature alias

[kevinjqliu]

Which issue does this PR close?

Closes #744 (and contributes to the 0.14.0 breaking release tracked in #657).

Rationale for this change

cloud was a back-compat alias for cloud-base + reqwest. Now that #724 introduced cloud-base, *-base, and an explicit reqwest feature, the cloud umbrella is redundant and ambiguous. Since 0.14.0 is already a breaking release, drop it cleanly rather than carry a deprecation alias.

What changes are included in this PR?

• Remove cloud = ["cloud-base", "reqwest"] from Cargo.toml.
• Remove the cloud row from the Feature Flags table in src/lib.rs docs.
• cloud-base and the *-base / aws / azure / gcp / http features
  are unchanged.

Are there any user-facing changes?

Yes — breaking. --features cloud now fails with the package 'object_store' does not contain this feature: cloud.

[kevinjqliu]

cc @kylebarron, dropping cloud feature 😄

do we need to add the api-change label?

[alamb]

Thanks @kevinjqliu and @kylebarron