Eleventy Fetch v5.1.0-beta.1

• Refetch data on transient network failures by @gorbak25 #57 #59
• Fix bug with partially purged cache files on CI servers #59
• Removes flat-cache dependency: 🏋🏻‍♀️ weight 1.6 MB to 1 MB and ✅ dependency count 18 to 11. #56
• Backwards compatibility with existing cache files is preserved (otherwise this would be a major breaking change).

Milestone: https://github.com/11ty/eleventy-fetch/milestone/6?closed=1
Full Changelog: v5.0.2...v5.1.0-beta.1