Remove unsupported platforms from asmdef

[jzapdot]

The latest SDK 4.3.1 doesn't compile for Unity 6000.3.11f1 when pulled from UPM. Instead:

• There are compilation errors due to the editor AssemblyDefinition package/Editor/io.sentry.unity.editor.asmdef not finding the runtime AssemblyDefinition package/Editor/Runtime/io.sentry.unity.runtime.asmdef.
• The runtime AssemblyDefinition package/Editor/Runtime/io.sentry.unity.runtime.asmdef doesn't load in the editor. In the inspector it shows a warning for invalid platforms.

I resolved these issues locally by customizing the package, removing the unsupported platforms, and toggling off the "Use Guids" checkbox on the package/Editor/Runtime/io.sentry.unity.runtime.asmdef and manually assigning the package/Editor/Runtime/io.sentry.unity.runtime.asmdef file.

This PR:

• Removes unsupported platforms 'Lumin' and 'Stadia' from the assembly definition.

I wasn't able to to determine why the package could not be assigned via guid. This is usually due to something being off with the meta guid assigned to the AsmDef and requires blowing away the meta and re-updating the AssemblyDefinition to have the original settings.

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[jzapdot]

This PR addresses issue #2689.