Fix some issues reported by lintian

Fix some issues reported by lintian

• Bump debhelper from deprecated 9 to 13. (package-uses-deprecated-debhelper-compat-version)
• Update standards version to 4.6.2, no changes needed. (out-of-date-standards-version)
• Avoid explicitly specifying -Wl,--as-needed linker flag. (debian-rules-uses-as-needed-linker-flag)

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/3d/21881a7a62acba0810e1430b9d5db6ec81dccf.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/71/655319b820f347578c422dbf89d2f2a622e2f6.debug

No differences were encountered between the control files of package **gridlock.app**

Control files of package gridlock.app-dbgsym: lines which differ (wdiff format)

• Build-Ids: 71655319b820f347578c422dbf89d2f2a622e2f6 3d21881a7a62acba0810e1430b9d5db6ec81dccf

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Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/gridlock.app/9ddeb95f-ad98-4ce4-bfb7-bda15b510764.