Add CI check for MSRV 1.77

[masanori0209]

Fixes #1449.

This adds a GitHub Actions job to verify that autocxx continues to build with the declared MSRV, Rust 1.77.

While adding the check, two small MSRV compatibility issues surfaced:

• build script cargo:: output is changed back to the Rust 1.77-compatible cargo: form.
• bindgen output is targeted at Rust 1.77 so generated bindings do not use newer Rust syntax.

Validation:

• cargo +1.77 check --workspace --all-targets
• cargo check --workspace --all-targets

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