Fix some issues reported by lintian

Fix some issues reported by lintian

• Use secure copyright file specification URI. (insecure-copyright-format-uri)
• Use secure URI in Homepage field. (homepage-field-uses-insecure-uri)
• Bump debhelper from deprecated 9 to 13. (package-uses-deprecated-debhelper-compat-version)
• Set debhelper-compat version in Build-Depends. (uses-debhelper-compat-file)
• Set upstream metadata fields: Bug-Database, Bug-Submit, Repository-Browse. (upstream-metadata-file-is-missing, upstream-metadata-missing-bug-tracking)
• Use canonical URL in Vcs-Browser, Vcs-Git. (vcs-field-not-canonical)
• Update standards version to 4.1.1, no changes needed. (out-of-date-standards-version)
• Set upstream metadata fields: Repository. (upstream-metadata-missing-repository)

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/22/0ef7823297fe4547bc41fa19ccad450635cdc4.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/39/3f73b334227661d26cf7e861a6fe79d48d2d2b.debug

Control files of package slic3r: lines which differ (wdiff format)

• Homepage: http​://slic3r.org/ https​://slic3r.org/

Control files of package slic3r-dbgsym: lines which differ (wdiff format)

• Build-Ids: 393f73b334227661d26cf7e861a6fe79d48d2d2b 220ef7823297fe4547bc41fa19ccad450635cdc4

You can also view the diffoscope diff (unfiltered).

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Build and test logs for this branch can be found at https://janitor.debian.net/run/d42ecc6e-991b-4fb6-a6d3-ab27d69876db.