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