[docker] Update DART version to 6.15 #230

Job	Run time
manylinux (min) (Dockerfile.manylinux_2_28_x86_64, quay.io/pypa/manylinux_2_28_x86_64, manylinux_...	38m 58s
manylinux (min) (Dockerfile.manylinux_2_28_aarch64-min, quay.io/pypa/manylinux_2_28_aarch64, many...	2h 6m 59s
manylinux (min) (Dockerfile.manylinux_2_28_ppc64le-min, quay.io/pypa/manylinux_2_28_ppc64le, many...	1h 58m 40s
manylinux (min) (Dockerfile.manylinux_2_28_s390x-min, quay.io/pypa/manylinux_2_28_s390x, manylinu...	1h 59m 6s
tracy profiler (v6.15, ON)	15m 38s
ubuntu (jammy, v6.15, linux/amd64,linux/arm64,linux/ppc64le,linux/s390x, OFF)	13m 37s
ubuntu (noble, v6.15, linux/amd64,linux/arm64,linux/ppc64le,linux/s390x, OFF)	12m 13s
ubuntu (oracular, v6.15, linux/amd64,linux/arm64,linux/ppc64le,linux/s390x, OFF)	12m 19s
manylinux (Dockerfile.manylinux_2_28_aarch64, jslee02/dart-dev:manylinux_2_28_aarch64-min-v6.15, ...	4h 27m 7s
manylinux (Dockerfile.manylinux_2_28_ppc64le, jslee02/dart-dev:manylinux_2_28_ppc64le-min-v6.15, ...	4h 22m 42s
manylinux (Dockerfile.manylinux_2_28_s390x, jslee02/dart-dev:manylinux_2_28_s390x-min-v6.15, many...	4h 18m 2s
	20h 45m 21s

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