Bump version for v1.15.39 release #248

Job	Run time
Run test and build (macos-14)	1m 3s
Run test and build (macos-15)	1m 2s
Run test and build (ubuntu-24.04-arm)	41s
Run test and build (macos-15-intel)	2m 37s
Run test and build (ubuntu-latest)	43s
	6m 6s

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