docs: match alpha.21 Debian release asset

[Lucenx9]

Summary

• align README download and install examples with the staged GitHub release asset name
• retain the Debian package internal version as 0.2.0~alpha.21

Verification

• cargo run -p xtask -- check
• git diff --check
• downloaded release asset and verified SHA256SUMS and dpkg metadata

This corrects the filename regression introduced while addressing the release PR review comment.

[coderabbitai]

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