Fix some issues reported by lintian

Fix some issues reported by lintian

• Remove 2 obsolete maintscript entries in 1 files.
• Bump debhelper from old 12 to 13. (package-uses-old-debhelper-compat-version)
• 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)
• Update standards version to 4.6.0, no changes needed. (out-of-date-standards-version)
• Avoid explicitly specifying -Wl,--as-needed linker flag. (debian-rules-uses-as-needed-linker-flag)
• Remove 2 obsolete maintscript entries in 1 files.

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-power-manager/c570105e-1279-4f15-8a2f-21bd02cd7421.

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/8b/c28ef0ab86395a3c72e74bc13d68076883ec68.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/ff/f55e2bf33d111a8420522e9797c04e33407760.debug
-rwxr-xr-x  root/root   DEBIAN/postinst
-rwxr-xr-x  root/root   DEBIAN/postrm
-rwxr-xr-x  root/root   DEBIAN/preinst
-rwxr-xr-x  root/root   DEBIAN/prerm

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

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

• Build-Ids: fff55e2bf33d111a8420522e9797c04e33407760 8bc28ef0ab86395a3c72e74bc13d68076883ec68

You can also view the diffoscope diff (unfiltered).