Upgrade to newer source format 3.0 (native).

Changes-By: lintian-brush
Fixes: lintian: missing-debian-source-format
See-also: https://lintian.debian.org/tags/missing-debian-source-format.html
Fixes: lintian: older-source-format
See-also: https://lintian.debian.org/tags/older-source-format.html