DAOS-16500 build: Move to Leap 15.6 (#15561) #7346

Job	Run time
Build DAOS (ARM64) (ubuntu, clang)	51m 48s
Build DAOS (alma.8)	36m 15s
Create Docker images (ubuntu)	8m 56s
Build DAOS (ARM64) (alma.8, clang)	35m 45s
Build DAOS (alma.9)	35m 47s
Create Docker images (rocky)	9m 47s
Build DAOS (rocky.8)	37m 37s
Build DAOS (ARM64) (leap.15, clang)	48m 28s
Create Docker images (fedora)	10m 14s
Build DAOS (ubuntu)	35m 12s
Create Docker images (leap.15)	11m 50s
Run DAOS/NLT tests (ubuntu)	24m 27s
Build DAOS in image (ubuntu, clang)	21m 30s
Build DAOS in image (ubuntu, gcc)	23m 4s
Build DAOS in image (rocky, clang)	21m 2s
Build DAOS in image (rocky, gcc)	22m 6s
Build DAOS in image (fedora, clang)	21m 45s
Build DAOS in image (fedora, gcc)	24m 3s
Build DAOS in image (leap.15, clang)	25m 12s
Build DAOS in image (leap.15, gcc)	23m 46s
	8h 48m 34s

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