Refresh the Godot sandbox demo

[limbonaut]

Refresh the Godot sandbox demo from the godot-4.5 branch:

• Upgrade to the latest stable Godot release and current Sentry Godot SDK.
• Replace the custom GameLogic extension with the crash triggers bundled in the Sentry Godot package.
• Move custom Godot builds from tools/ to versioned GitHub Releases in getsentry/godot-builds.
• Remove other large generated binaries and debug symbols from the demo repository.
• Pin and download required build artifacts during setup or CI.
• Update CI to download the matching standalone debug-symbol package from the Sentry Godot SDK release and use it when uploading symbols to Sentry.
• Update the sandbox documentation and CI workflow.
• Verify the refreshed demo flows.
• Promote the refreshed project to the default branch.
• Review the SDK features added over the past year and select which ones the sandbox should showcase.

[linear-code]

GODOT-243