Sanitize the summary field

Thanks! The codes read sane, I would like to check if we can add some tests for this somehow, will get back to this soon, unless you already have an idea for the testing.

Made a MR of the patch and added testing to check if HTML is stripped from the summary field.

Updated this some, including adding tests for 10.5 (previous stable). This is work @Jeroent did over at 📌 Automated Drupal 11 compatibility fixes for ics_field Needs review but I thought I would like to use it here already.

Code changes look good, requested tests were added, going to commit this.

Automatically closed - issue fixed for 2 weeks with no activity.