Fix 3.38 usage

.gitlab-ci.yml

-# SPDX-FileCopyrightText: 2018, 2020 Bruce Cowan <bruce@bcowan.me.uk>
+# SPDX-FileCopyrightText: 2018-2021 Bruce Cowan <bruce@bcowan.me.uk>
+#
 # SPDX-License-Identifier: CC0-1.0
 flatpak:
   image: fedora:latest
@@ -14,8 +15,8 @@ flatpak:
   script:
   - dnf install -y flatpak-builder
   - flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
-  - flatpak install -y flathub org.gnome.Platform 3.36
-  - flatpak install -y flathub org.gnome.Sdk 3.36
+  - flatpak install -y flathub org.gnome.Platform 3.38
+  - flatpak install -y flathub org.gnome.Sdk 3.38
   - flatpak-builder --stop-at=${FLATPAK_MODULE} app ${MANIFEST_PATH}
   - flatpak build app meson --prefix=/app ${MESON_ARGS} _build
   - flatpak build app ninja -C _build install