Prevent deprecated tools from rendering as broken links

[mre]

This PR fixes the remaining link checker failures by ensuring deprecated tools render as bold text instead of clickable links.

Problem

After PR #1692 fixed broken URLs by updating working tools and marking others as deprecated, the link checker was still failing because deprecated tools were still being rendered as clickable links like [BugProve](https://broken-url.com) in the README.

Root Cause

Two issues prevented proper handling of deprecated tools:

1. Template Logic: The README template rendered all tools as links regardless of deprecated status
2. Backfill Override: The backfill logic was overwriting manually set deprecated: true flags with null values from the old JSON data

Solution

Template Changes (data/render/templates/README.md)

• Added conditional rendering: deprecated tools show as **Tool Name** (bold text) instead of [Tool Name](url) (clickable link)
• All deprecated tools retain the :warning: icon for clear visual indication
• Removed broken sponsor links from template footer

Code Changes (data/render/src/bin/main.rs)

• Fixed backfill logic to only backfill deprecated status when not already explicitly set in YAML
• Prevents overwriting manually set deprecated: true flags

Results

Before

After

Testing

• ✅ All deprecated tools now render as bold text without links
• ✅ Non-deprecated tools still render as proper clickable links
• ✅ Warning icons still show for deprecated tools
• ✅ Link checker should now pass without broken URL errors

Tools Fixed

The following deprecated tools no longer render as broken links:

• BugProve, steady, HasMySecretLeaked, Betterscan CE, kubeval, Scrutinizer

This complements the URL fixes from PR #1692 and provides a sustainable solution for future deprecated tools.