codegen-rustfmt: use 2021 edition #2931

Job	Run time
clippy	1m 46s
license-guard	30s
godot-itest (macos)	1m 6s
doc-lints	57s
godot-itest (windows)	2m 11s
run-examples	1m 46s
rustfmt	23s
godot-itest (linux-full)	1m 50s
unit-test	2m 23s
godot-itest (linux-features-experimental)	2m 19s
godot-itest (linux-4.1)	1m 25s
godot-itest (linux-4.2)	1m 9s
godot-itest (linux-memcheck)	2m 29s
ci-status	1s
	20m 15s

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