metainfo: Fix validate error and deprecation; shorten OARS

[ryonakano]

• Add launchable tag, which fixes the appstreamcli validation error:

[ryo@b760m ~/work/tmp/harvey (main =)]$ ninja -C builddir
ninja: Entering directory `builddir'
[1/1] Merging translations for data/io.github.danirabbit.harvey.appdata.xml
[ryo@b760m ~/work/tmp/harvey (main =)]$ appstreamcli validate builddir/data/io.github.danirabbit.harvey.appdata.xml
E: io.github.danirabbit.harvey:~: desktop-app-launchable-missing

✘ Validation failed: errors: 1, pedantic: 1
[ryo@b760m ~/work/tmp/harvey (main =)]$

• Replace deprecated SPDX identifier GPL-3.0+ with GPL-3.0-or-later
• Shorten OARS metadata