fix(release): only show "New Contributors" for genuine first-timers

[Fu-Jie]

Problem

The release workflow's contributor-attribution step injected a ## New Contributors section for any external contributor, not just first-time ones. The first_time flag only changed the bullet wording ("made their first contribution 🎉" vs "contributed in"), but the section header always said "New Contributors".

This caused returning contributors to be mislabeled as new. Example: @rbb-dev contributed #103 (released in release-2026.07.02), then #104 — but release-2026.07.09 listed them under "New Contributors" again.

Fix

Only emit the ## New Contributors section when CONTRIBUTOR_FIRST_TIME=true. Returning contributors are already credited via their PR in the changelog and don't need a separate (incorrectly-labeled) callout.

Before:

if [ "$CONTRIBUTOR_IS_EXTERNAL" = "true" ] && ...; then
    echo "## New Contributors" >> release_notes.md   # ← always "New Contributors"
    if [ "$CONTRIBUTOR_FIRST_TIME" = "true" ]; then
        echo "* @... made their first contribution ..."   # first-timer
    else
        echo "* @... contributed in #..."                 # returning (wrong header!)
    fi
fi

After:

if [ "$CONTRIBUTOR_IS_EXTERNAL" = "true" ] && [ "$CONTRIBUTOR_FIRST_TIME" = "true" ] && ...; then
    echo "## New Contributors" >> release_notes.md
    echo "* @... made their first contribution in #... 🎉"
fi

Already-fixed artifact

The already-published release-2026.07.09 body had the incorrect section; it was edited in-place to remove the ## New Contributors block (rbb-dev is not new — #103 was their first, in release-2026.07.02).

Verification

• release-2026.07.02 (rbb-dev Restore DOCX & XLSX export on Open WebUI 0.10.x (+ opt-in external images) #103): correctly says "made their first contribution in Restore DOCX & XLSX export on Open WebUI 0.10.x (+ opt-in external images) #103 🎉" ✅
• release-2026.07.09 (rbb-dev fix(export_to_docx): render Mermaid diagrams with <br> labels — v0.5.2 #104): now has no "New Contributors" section ✅
• Future returning contributors: no longer mislabeled ✅
• Future genuine first-timers: still get the callout ✅

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