build-aux,CI: Update KDE Flatpak runtime to 6.8

The underlying Freedesktop runtime is now 24.08 with AOM 3.10.x and
libvpx 1.14.x

.github/workflows/build-project.yaml

@@ -249,7 +249,7 @@ jobs:
     env:
       FLATPAK_BUILD_SHARE_PATH: flatpak_app/files/share
     container:
-      image: ghcr.io/flathub-infra/flatpak-github-actions:kde-6.6
+      image: ghcr.io/flathub-infra/flatpak-github-actions:kde-6.8
       options: --privileged
     steps:
       - uses: actions/checkout@v4

.github/workflows/publish.yaml

@@ -62,7 +62,7 @@ jobs:
       YOUTUBE_SECRET: ${{ secrets.YOUTUBE_SECRET }}
       YOUTUBE_SECRET_HASH: ${{ secrets.YOUTUBE_SECRET_HASH }}
     container:
-      image: ghcr.io/flathub-infra/flatpak-github-actions:kde-6.6
+      image: ghcr.io/flathub-infra/flatpak-github-actions:kde-6.8
       options: --privileged
     strategy:
       matrix: