Use GitHub tags page in debian/watch

This allows building packages after the tag was pushed but before the corresponding GitHub release was published.

The same change was already done in deb-icinga2:
deb-icinga2@e8e9e552

debian/watch

@@ -2,4 +2,4 @@ version=3
 # filenamemangle -> prepending icinga2 if only tag name
 # versionmangle  -> convert version addition to ~ version
 opts=filenamemangle=s/.+\/(?:icinga2-|v)([\d.]+(?:-.+)?).tar.gz/icinga2-$1.tar.gz/,versionmangle=s/^([\d.]+)-(.+)?/$1~$2/ \
-https://github.com/Icinga/icinga2/releases .*/(?:icinga2-|v)([\d.]+(?:-.+)?).tar.gz
+https://github.com/Icinga/icinga2/tags .*/(?:icinga2-|v)([\d.]+(?:-.+)?).tar.gz