Update to the Freedesktop 26.08 runtime

Please consider updating the Steam Flatpak to the Freedesktop 26.08 runtime when it becomes available and compatible.

The main motivation is to keep Mesa up to date, improving graphics-driver support and compatibility for Steam games, especially on systems that rely on the runtime-provided Mesa stack.

Additional context

Current runtime: Freedesktop 25.08 currently used by the manifest
Requested runtime: org.freedesktop.Platform//26.08
Motivation: obtain a newer Mesa version and related graphics improvements

Thanks for maintaining the Steam Flatpak!

Start by locating the Steam Flatpak manifest and inspect its current Freedesktop 25.08 runtime declaration. Check whether org.freedesktop.Platform//26.08 is available and compatible, then update the runtime reference and verify that the Flatpak builds with the newer Mesa-providing runtime.

Written by the indexing model from the issue text.