Clarify whether LSe is licensed under GPL-3.0-Only or GPL-3.0-Or-Later

[sanana]

Source code headers don't seem to clarify this, from a legal standpoint GPL-3.0-Only is the default variant of the GPL3 and if a newer revision is released it would make LSe incompatible with software licensed under the newer revision. Moreover it's useful for packagers to know the variant of the GPL that LSe is licensed under.

Note: I am not a lawyer