Use source mtime for Last-Modified and unify modified-since semantics #2560

Job	Run time
docker_build (linux/arm/v7, minimal)	1m 4s
docker_build (linux/arm64, minimal)	54s
docker_build (linux/arm64, duckdb)	51s
hurl_examples	7s
docker_build (linux/amd64, duckdb)	4m 43s
docker_build (linux/amd64, minimal)	4m 32s
compile_and_lint	2m 13s
windows_test	1m 47s
playwright	0s
test	0s
hurl (${{ matrix.example }})	0s
docker_push	0s
	16m 11s

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