Fix some issues reported by lintian

Fix some issues reported by lintian

• Set upstream metadata fields: Bug-Database, Bug-Submit, Contact, Repository, Repository-Browse. (upstream-metadata-file-is-missing, upstream-metadata-missing-bug-tracking, upstream-metadata-missing-repository)
• 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 (out-of-date-standards-version)
• Set field Upstream-Contact in debian/copyright.
• Remove obsolete field Contact from debian/upstream/metadata (already present in machine-readable debian/copyright).

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-boxes/876d0dd7-c464-46c9-8b08-12553f6c4bfd.

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/b0/5f3d29cad59d06d51bcc875dd31ed6fe237542.debug
-rw-r--r--  root/root   /usr/lib/debug/.build-id/c2/ebadce7f0da2567d2484bc2ce93d12d5bec7ce.debug
-rw-r--r--  root/root   /usr/lib/debug/.build-id/cd/46305ae99fc27e1f7edebd7e776bbb35e74417.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/21/6553024c12d0df8aceceecf78b1a65fa904f9b.debug
-rw-r--r--  root/root   /usr/lib/debug/.build-id/38/99fbd88bfbd54b49576374e2252a9026c3b62d.debug
-rw-r--r--  root/root   /usr/lib/debug/.build-id/4b/83356c6c7e8a11b741ea684a32264556097659.debug

No differences were encountered between the control files of package **gnome-boxes**

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

• Build-Ids: 216553024c12d0df8aceceecf78b1a65fa904f9b 3899fbd88bfbd54b49576374e2252a9026c3b62d 4b83356c6c7e8a11b741ea684a32264556097659 b05f3d29cad59d06d51bcc875dd31ed6fe237542 c2ebadce7f0da2567d2484bc2ce93d12d5bec7ce cd46305ae99fc27e1f7edebd7e776bbb35e74417

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