observium-agent: Update to version 23.1

observium-agent: Update to version 23.1

- Update from version 18.9.1 to 23.1
- Update of rootfile not required
- Changelog - There is no changelog file inb the source tarball.
   The commit changes are done in a SVN change management system. There is a Changelog
   page on the website, https://changelog.observium.org/, but this identifies all changes
   by their SVN number but with no relationship to the release version number.
- I have not been able to find out how to identify what changes have been made between
   version 18.9.1 and 23.1

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>