Fix metainfo validation

[dbnicholson]

Currently the metainfo validation is failing as in this build:

>>> MALLOC_PERTURB_=85 /usr/bin/appstream-util validate --nonet data/metainfo/org.endlessos.Key.Devel.metainfo.xml
――――――――――――――――――――――――――――――――――――― ✀  ―――――――――――――――――――――――――――――――――――――
stdout:
data/metainfo/org.endlessos.Key.Devel.metainfo.xml: FAILED:
• style-invalid         : <caption> is too long [Bring equity into the classroom; curated learning anywhere, anytime with Endless Key];longest allowed is 50 chars
• style-invalid         : <caption> is too long [Completely free; access without an account, safeguarded data, no ads, and no third-party involvement.];longest allowed is 50 chars
• style-invalid         : <caption> cannot end in '.' [Completely free; access without an account, safeguarded data, no ads, and no third-party involvement.]
• style-invalid         : <caption> is too long [Engaging and interactive; discover an abundance of ready-made resources, interactive games, simulations, and a vast ocean of curated content.];longest allowed is 50 chars
• style-invalid         : <caption> cannot end in '.' [Engaging and interactive; discover an abundance of ready-made resources, interactive games, simulations, and a vast ocean of curated content.]
• style-invalid         : <caption> is too long [Grades 5–9, 2700+ curated resources, 1500+ videos, 300+ e-books];longest allowed is 50 chars
• style-invalid         : <caption> is too long [Resources such as finance, healthy body, TED Ed, sports, games, arts & crafts, gardening, and more];longest allowed is 50 chars
• style-invalid         : <caption> is too long [Works online and offline; access learning resources for coding, STEM, cooking, and arts & crafts! Videos, games, and activities included. No Internet required!];longest allowed is 50 chars
stderr:
Validation of files failed

@pwithnall said he was looking at metainfo validation, which I presume means changing from appstream-util to appstreamcli. So, it's possible the validation errors will go away or be different. However, if the above points are valid, then we'll need some new wording. CC @cassidyjames.

[dbnicholson]

See #33.