[3391] Accept gradient for node background

[frouene]

Bug: #3391

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[sbegaudeau]

I don't mind this change at all but we have to be consistent. Could you change the GraphQL schema and the background code to match this new behavior please? You may also need a migration participant in sirius-web-application to match the new name.

[frouene]

Wait for migration participant to be merged to add a new one

[frouene]

A new participant migration has been add