fix(composer): declare ext-xsl so dependabot can resolve (#884)

Dependabot cannot propose composer updates for this app. Its updater fails
with:

  Your requirements could not be resolved to an installable set of packages.
    - edgedesign/phpqa[v1.27.0, ..., v1.27.2] require ext-xsl * ->
      it is missing from your system.

config.platform pins php 8.3 so composer resolves against a known PHP version,
but says nothing about extensions. edgedesign/phpqa requires ext-xsl and the
resolving environment does not have it, so the resolve fails before any bump
can be computed.

CI is unaffected, which is why this went unnoticed: composer install replays
the committed lock and never re-resolves, so the pipeline stays green while
dependabot -- which does re-resolve -- fails every time. The shared quality.yml
installs no xsl extension anywhere and never invokes phpqa.

Declaring ext-xsl beside the php pin makes resolution assume exactly what the
committed lockfile already assumes. Verified in a clean composer:2 container
without --ignore-platform-reqs: the same file fails without this line and
resolves (103 installs, lock written) with it.

Co-authored-by: Conduction Release Bot <release-bot@conduction.nl>

Author: rubenvdlinde

composer.json

@@ -104,7 +104,8 @@
         "optimize-autoloader": true,
         "sort-packages": true,
         "platform": {
-            "php": "8.3"
+            "php": "8.3",
+            "ext-xsl": "1"
         }
     }
 }