fix: generate release changelog from built ISO artifacts

[hcastc00]

Summary

• Update the release workflow so CHANGELOG.md uses the ISO filenames produced by the build jobs instead of hardcoded Bookworm names.
• Read Debian and Ubuntu SHA-256 files into release-ready lines without the local images/ path prefix.
• Replace the long echo -en release body with a heredoc for easier maintenance.
• Update the README release asset examples to match the current attended and unattended ISO naming pattern.

Why

The release body was still assuming fixed DAppNode-...bookworm... ISO names, while the workflow now renames generated artifacts with distro/version-specific Dappnode-...-attended.iso and Dappnode-...-unattended.iso filenames. Deriving the release content from the generated artifacts keeps GitHub release notes aligned with the files actually uploaded.

Test plan

• Run the release workflow and verify the generated release body lists the exact Debian attended, Debian unattended, and Ubuntu unattended ISO filenames.
• Verify each checksum block matches the uploaded ISO filename and contains no local images/ prefix.
• Confirm the README release asset examples match the published artifact naming pattern.