Create one issue per vulnerability, and prevent duplicates

[facutuesca]

Description

This changes the GH Actions workflow so that a new issue is created for each new vulnerability detected. If an issue already exists, it won't create a new one.
This prevents the current issue, where a new issue will be created each day with the same vulnerabilities, until the script stops reporting them.

Screenshot

How it works:

• The output of the script is parsed to extract all the vulnerabilities IDs
• Those IDs are inserted into a matrix, so that one job will be run for each
• The new job create-issues will be run for each vulnerability and create an issue with the title New vulnerability CVE-XXXX-YYYYY found on main
• If an issue with the same title already exists (open or closed), it won't create a new issue

[mhdawson]

LGTM