Update to Flatpak runtime 24.08

[TobiX]

Runtime 23.08 is EOL since this month.

Update LLVM from 18 to 20 at the same time.

I uses LLVM 20, even if a LLVM 21 SDK is availaibe, since that is the newest version available on the GitLab runners, which makes it easy to keep the Flatpak & Linux builds in sync. I can split out the LLVM 20 update into a new PR if you want.

[blueskythlikesclouds]

LLVM update should be its own PR. We unfortunately cannot upgrade the compiler version with our eyes closed, as we had experiences with clang updates breaking recompiled code behavior in the past.

[TobiX]

I'll convert this to an update to last year's runtime release then (24.08), which is still supported until September 2026 (see https://freedesktop-sdk.gitlab.io/documentation/updating-sdk/release-notes/) and the LLVM 18 extension is still available for that release.

This also makes it easier to update to LLVM 20 later, since that extension is available for 24.08 and 25.08.

[TobiX]

Others (Heroic, Lutris, Steam?) seem to have issues with controller input on SteamDeck with runtimes newer then 23.08, I'll test on SteamDeck later. (But this might be Wine/Proton-related, so probably not relevant for native applications)

Relevant issues:

• Steam Deck input not working with Flatpak Runtime 24.08+ (with workaround) Heroic-Games-Launcher/HeroicGamesLauncher#4708
• https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/issues/1818
• Steam Deck controller doesnt work after Lutris flatpak update, possibly runtime related. flathub/net.lutris.Lutris#467

[TobiX]

I'll test on SteamDeck later.

Tested a local build from my PR. Works fine in game mode.