xfce4-datetime-plugin: update to 0.8.1

Change log:

datetime-0.8.1
======
- Update configure.ac.in syntax and enable debug when building from git
- Remove GSourceFunc casts
- Fix font size (#30)
- Add new README.md and update AM_INIT_AUTOMAKE
- Add basic GitLab pipeline
- Update URLs from goodies.x.o to docs.x.o (Bug #16159)
- s/http/https
- Update .gitignore
- Fix build with panel 4.15
- Make build less verbose
- Translation Updates:
  Albanian, Armenian (Armenia), Belarusian, Eastern Armenian, English
  (Canada), Estonian, Galician, Hebrew, Interlingue, Persian (Iran),
  Slovak, Spanish, Turkish, Uyghur, Uzbek

(gutteridge)