Fix Antora prerelease append newline

[jruaux]

Follow-up to #8. Ensure prerelease is appended on a new line when docs/antora.yml does not already end with a trailing newline.

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Note

Low Risk
Low risk: a one-line change to the GitHub composite action’s printf ensures prerelease is appended with a leading newline, affecting only docs version metadata updates.

Overview
Fixes the Antora version update GitHub Action so that when prerelease is missing from docs/antora.yml, it is appended with a leading newline (\nprerelease: ...) to avoid being concatenated onto the last line when the file lacks a trailing newline.

^{Written by Cursor Bugbot for commit 7dc4de8. This will update automatically on new commits. Configure here.}

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}