Update MSRV to 1.89.0 to match AtCoder

[Copilot]

Update Rust version to 1.89.0 for AtCoder compatibility

• Update CI workflow (.github/workflows/ci.yml) to use Rust 1.89.0 instead of 1.70.0
• Update rust-version in proconio/Cargo.toml from 1.70.0 to 1.89.0
• Update UI test version check in proconio/tests/ui.rs from 1.70.0 to 1.89.0
• Update related comments in Cargo.toml about MSRV requirements
• Remove unnecessary winapi-util override (required only Rust 1.71+, now MSRV is 1.89.0)
• Remove backtrace version pin (no longer needed with MSRV 1.89.0)
• Run CI tests to validate changes (cargo build, test, clippy, and fmt all pass)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fix CI errors</issue_title>
<issue_description>Rust 1.89.0 is used in AtCoder now. We need to update it in the CI too.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Fix CI errors #61

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[statiolake]

@copilot Do we still need to pin backtrace? If it passes without pinning, let's remove that.

[Copilot]

Removed in 281f7b2. Tests pass successfully with the latest backtrace v0.3.76.