chore(alpine): add eol date for Alpine 3.20

[DmitriyLewen]

Description

Add eol date for Alpine 3.20.
trivy-db already contains vulns for 3.20.

➜ trivy image alpine 
...
2024-05-28T13:19:53+06:00       INFO    Detected OS     family="alpine" version="3.20.0"
2024-05-28T13:19:53+06:00       INFO    [alpine] Detecting vulnerabilities...   os_version="3.20" repository="3.20" pkg_num=14

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).