Standardize copyright headers. #5776

Job	Run time
cargo fmt	19s
druid doctests	1m 16s
mdbook build	1m 46s
Docs (windows-latest)	2m 28s
cargo clippy+test (windows-latest)	3m 52s
cargo clippy+test (gtk)	3m 44s
Docs (macos-latest)	1m 52s
cargo test (wasm32)	2m 34s
cargo clippy+test (macos-latest)	2m 58s
Docs (ubuntu-latest)	3m 47s
cargo clippy+test (ubuntu-latest)	3m 21s
cargo clippy+test (wayland)	2m 53s
	30m 50s

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