[icalendar] gracefully ending maintenance

[daMihe]

PR will close this issue automatically, this issue is created just to name branch properly.

I'm ending the maintenance of icalendar as i don't use openHAB anymore and so don't have any motivation to fix upcoming issues anymore. Partly an issue is also i had to wait nearly a year for a review on other pull requests which also just takes out the fun of programming there.

I'm doing this gracefully offering anybody to take over the maintenance and the open pull requests by just cloning my fork (which will be archived).