devel/appstream: update to 0.15.3

This release contains mostly a bunch of bugfixes, but also cleans up
the Meson build system code and now requires Meson >= 0.62, which
isn't great for backports, but is great to simplify maintenance of
AppStream's code.
I do hope that this change will not cause too many problems (most
distributions seem to backport Meson for other software already).

Tarballs available here:
https://www.freedesktop.org/software/appstream/releases/

Version 0.15.3
~~~~~~~~~~~~~~
Released: 2022-04-10

Features:

• qt: Include enums for VcsBrowser and Contribute
• Add vcs-browser and contribute URL type
• validate: Improve validation of desktop files alongside metainfo data
• its: Mark deprecated rules as deprecated
• Implement l10n support for metainfo keyword tags
• validator: Perform basic validation of keywords in metainfo data
• compose: Prefer metainfo keywords over desktop-entry keywords
• meson: Bump minimum version to 0.62

Specification:

• docs: Document keywords for metainfo files as well
• docs: Spell it metainfo, not meta-info if referencing metainfo.xml files
• spec: Document how keywords in metainfo files should be translated

Bugfixes:

• qt: If the timestamp is 0, return a default QDateTime()
• docs: Include compose manual page
• validate: Point at the right line for description-enum-item-invalid
• validator: Find data if /usr prefix is missing as well
• validator: Make insufficient launch data for desktop-apps a hard error
• Don't fail downloads or URL checks if redirects were involved
• apt: Only refresh the OS data cache (not the whole system cache) on

APT update

• compose: Emit error if filters are set but no output was generated
• Centralize GOnce guard to fix an assertion failure in AsComponent

Contributors:
Aleix Pol, JakobDev, Matthias Klumpp, Simon McVittie