[Flatpak] Update manifest for UMU support

[R1kaB3rN]

Related to #3480 and depends on the updated Flatpak support in ULWGL aka umu to be merged/reviewed. After the Flatpak umu is installed in the user's system, the Heroic Games Launcher will need to allow explicit access to umu's $XDG_DATA_HOME/umu and the system /app/share directory to run games through the launcher.

The reason is because, without going into the details, umu will currently copy files from /app/share to its private Flatpak directory, and Valve's Steam Runtime tools (e.g., Pressure Vessel) are designed to only be executed from the user's home directory. Without explicit visibility to these directories defined in Heroic's manifest, it's likely that umu will fail to launch games successfully due to Flatpak's strict filesystem permissions.

---

Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[flavioislima]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[R1kaB3rN]

I have read the CLA Document and I hereby sign the CLA

[GloriousEggroll]

umu flatpak update has been merged:
Open-Wine-Components/umu-launcher#82

However please note this PR needs modification now as the umu-launcher manifest has been renamed from:

org.openwinecomponents.umu.launcher

to:

org.openwinecomponents.umu.umu-launcher

[flavioislima]

umu flatpak update has been merged: Open-Wine-Components/umu-launcher#82

However please note this PR needs modification now as the umu-launcher manifest has been renamed from:

org.openwinecomponents.umu.launcher

to:

org.openwinecomponents.umu.umu-launcher

Thanks for the letting us know 👍🏽

[CommandMC]

I don't think the manifest kept in this repo is that relevant? It's only used when installing a local test build.
I've opened flathub/com.heroicgameslauncher.hgl#153 to add these changes to the Flathub manifest