Fix some issues reported by lintian

Fix some issues reported by lintian

• Set upstream metadata fields: Bug-Database, Bug-Submit, Contact, Repository-Browse. (upstream-metadata-file-is-missing, upstream-metadata-missing-bug-tracking)
• Update standards version to 4.6.1, no changes needed. (out-of-date-standards-version)

This merge proposal was created by the Janitor bot, and it will automatically rebase or close this proposal as appropriate when the target branch changes. Any comments you leave here will be read by the Janitor's maintainers.

Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/gnome-nettool/153fbd4a-07ec-42a4-881f-db3aba0d45dc.

Debdiff

These changes affect the binary packages:

[The following lists of changes regard files as different if they have different names, permissions or owners.]

Files in second set of .debs but not in first

-rw-r--r--  root/root   /usr/lib/debug/.build-id/cd/e65d3409deec51824b1f23011a959d1a109789.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/6b/4868c272b984c1af6b25ad87941c7f8be32a5e.debug

Control files of package gnome-nettool: lines which differ (wdiff format)

• Depends: libc6 (>= 2.28), 2.34), libcairo2 (>= 1.2.4), libgdk-pixbuf-2.0-0 (>= 2.22.0), libglib2.0-0 (>= 2.26.0), libgtk-3-0 (>= 3.0.3), libgtop-2.0-11 (>= 2.22.3), libpango-1.0-0 (>= 1.14.0), dconf-gsettings-backend | gsettings-backend, dnsutils, finger, iputils-ping | ping, iputils-tracepath, net-tools, nmap, whois

Control files of package gnome-nettool-dbgsym: lines which differ (wdiff format)

• Build-Ids: 6b4868c272b984c1af6b25ad87941c7f8be32a5e cde65d3409deec51824b1f23011a959d1a109789

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