refactor: remove redundant ICU version-detection mechanism

[clydegerber]

Summary

The workspace's minimum-supported ICU version is 74 (per the CI matrix: 74, 76, 77), so the icu_version_64_plus, icu_version_67_plus, and icu_version_68_plus gates are always active in practice. This PR removes them entirely along with the supporting infrastructure.

Removes:

• Redundant #[cfg(feature = \"icu_version_##_plus\")] annotations and their companion not(...) dead-code branches from rust_icu_ecma402, rust_icu_ulistformatter, rust_icu_unumberformatter, and rust_icu_uloc, including the pre-ICU-67 buggy-behavior test variant.
• The icu_version_64_plus, icu_version_67_plus, icu_version_68_plus, and icu_version_69_max Cargo feature declarations across 27 manifests. (icu_version_69_max was already a ghost — declared widely but never consumed by any source code.)
• The cfg-emission blocks in rust_icu_sys/build.rs and rust_icu_release/src/lib.rs.
• The _plus features from the test-with-features CI matrix and the macos-test Makefile target.

Rewrites the CONTRIBUTING.md section as forward-looking guidance for re-introducing version-dependent code if it becomes necessary again.

Alternative approach considered

An alternative would be to keep the version-gated code intact and add build.rs files to the few crates that lack auto-detection of the ICU version (rust_icu_ecma402, rust_icu_ulistformatter, rust_icu_unumberformatter). That would preserve the public Cargo feature surface but leaves dead conditional-compilation in place — every supported ICU version satisfies the gates, so the not(...) branches are unreachable and the _plus features are activated 100% of the time. Removing the mechanism outright eliminates the dead code and simplifies the build graph; if a future ICU version ever introduces a real boundary, the pattern can be re-introduced (see CONTRIBUTING.md).

Breaking change

This removes Cargo features from the public surface of every wrapper crate. The next release already requires a major version bump for unrelated reasons (the recent PartialEq derive removals from bindgen-generated types and the Text::try_from ownership-behavior change), so this fits the breaking-changes batch.

Test plan

• cargo test passes on default features (217 tests)
• CI test-with-features matrix passes with the updated feature list

This commit was created by an automated coding assistant, with human supervision.

[clydegerber]

This is the alternative to PR 363.