Update Target / WantedBy in systemd service definitions

Running lintian on our Debian package after pulling in the service definitions results in lint errors about our Target and WantedBy values

W: tinypilot: ]8;;https://lintian.debian.org/tags/systemd-service-file-refers-to-obsolete-targetsystemd-service-file-refers-to-obsolete-target]8;; syslog.target [lib/systemd/system/update-tls-cert-common-name.service]
W: tinypilot: ]8;;https://lintian.debian.org/tags/systemd-service-file-refers-to-obsolete-targetsystemd-service-file-refers-to-obsolete-target]8;; syslog.target [lib/systemd/system/usb-gadget.service]
W: tinypilot: ]8;;https://lintian.debian.org/tags/systemd-service-file-refers-to-unusual-wantedby-targetsystemd-service-file-refers-to-unusual-wantedby-target]8;; local-fs.target [lib/systemd/system/update-tls-cert-common-name.service]
W: tinypilot: ]8;;https://lintian.debian.org/tags/systemd-service-file-refers-to-unusual-wantedby-targetsystemd-service-file-refers-to-unusual-wantedby-target]8;; local-fs.target [lib/systemd/system/usb-gadget.service]

Exited with code exit status 2

These have always been the values we've used, so this isn't urgent, but maybe we can find better ones.

Start with lib/systemd/system/update-tls-cert-common-name.service and lib/systemd/system/usb-gadget.service, then review the linked lintian tags to understand acceptable Target and WantedBy values. Run lintian on the Debian package after updating both service definitions; done means the reported systemd target warnings are gone.