treewide: Update license headers to GPL-3.0-or-later and more bits of !606
This moves over some bits from !606 (merged) using 'per topic' rather than 'per file' commits - !606 (merged) rebases cleanly on top of this.
Edited by Guido Gunther
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This moves over some bits from !606 (merged) using 'per topic' rather than 'per file' commits - !606 (merged) rebases cleanly on top of this.