Use HTTPS MDPI link and increase urlchecker timeout.

5 jobs for master in 18 minutes and 37 seconds (queued for 3 seconds)
Name Stage Failure
failed
test_urls Test
 /builds/EnMAP/GFZ_Tools_EnMAP_BOX/EnPT/.github/ISSUE_TEMPLATE.md 
----------------------------------------------------------------
No urls found.

🤔 Uh oh... The following urls did not pass:
https://www.mdpi.com/2072-4292/9/7/676
make: *** [Makefile:66: urlcheck] Error 1
Cleaning up file based variables
ERROR: Job failed: exit code 1