[META] Roadmap for stable release

Problem/Motivation

It would be great if we can have a stable release for this project. This will also allow to add security coverage for the module.

Remaining tasks

• #3208587: Add caching calendar generation service
• #3208590: Add Tests coverage
• #3208578: Update eluceo/ical library to 0.18.x for stable release.
• (Nice-to-have)#3208597: [META] More options/flexibility about multi-day events, support repeated events

Separate issues will be created with more info for all tasks needed.