docs: bump README Rust requirement from 1.93.0 to 1.96.1 (fixes #5781)

[dominikake]

Summary

Fixes #5781. The README "Contributing from source" instructions tell contributors to install Rust 1.93.0, but rust-toolchain.toml pins channel = "1.96.1" (required by rusqlite 0.40 / libsqlite3-sys 0.38 via cfg_select!). Following the README leads to build failures.

Updated all 6 README versions to match the toolchain pin and CONTRIBUTING.md:

• README.md
• docs/README.zh-CN.md
• docs/README.de.md
• docs/README.ja-JP.md
• docs/README.ko.md
• docs/README.ur-pk.md

Acceptance criteria

• All 6 READMEs say Rust 1.96.1
• Version matches rust-toolchain.toml channel and CONTRIBUTING.md prerequisite table

Verification

grep -rn "1.93.0" README.md docs/README.*.md → no matches; all 6 now read Rust 1.96.1.

Disclosure

This PR was generated with the assistance of an AI coding agent (opencode) and reviewed by the human contributor before submission.

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[dominikake]

Closing as duplicate of #5798 (same #5781 fix, opened earlier). Will focus on less-contested issues instead.