Add Python 3.12 to the CPU test matrix #13606

Job	Run time
pl-cpu (macOS-14, lightning, 3.10, 2.1)	13m 43s
pl-cpu (ubuntu-20.04, lightning, 3.10, 2.1)	14m 53s
pl-cpu (windows-2022, lightning, 3.10, 2.1)	17m 6s
pl-cpu (macOS-14, lightning, 3.11, 2.2)	14m 5s
pl-cpu (ubuntu-20.04, lightning, 3.11, 2.2)	15m 6s
pl-cpu (windows-2022, lightning, 3.11, 2.2)	15m 31s
pl-cpu (macOS-14, lightning, 3.11, 2.3)	14m 25s
pl-cpu (ubuntu-20.04, lightning, 3.11, 2.3)	15m 32s
pl-cpu (windows-2022, lightning, 3.11, 2.3)	17m 3s
pl-cpu (macOS-14, lightning, 3.12, 2.4)	14m 40s
pl-cpu (ubuntu-20.04, lightning, 3.12, 2.4)	19m 28s
pl-cpu (windows-2022, lightning, 3.12, 2.4)	16m 33s
pl-cpu (macOS-12, pytorch, 3.10, 2.1)	22m 44s
pl-cpu (ubuntu-22.04, pytorch, 3.10, 2.1)	14m 28s
pl-cpu (windows-2022, pytorch, 3.10, 2.1)	17m 24s
pl-cpu (macOS-13, lightning, 3.9, 2.1, oldest)	21m 23s
pl-cpu (ubuntu-20.04, lightning, 3.9, 2.1, oldest)	13m 48s
pl-cpu (windows-2022, lightning, 3.9, 2.1, oldest)	15m 43s
pl-cpu (macOS-14, pytorch, 3.9, 2.1)	14m 9s
pl-cpu (ubuntu-20.04, pytorch, 3.9, 2.1)	15m 32s
pl-cpu (windows-2022, pytorch, 3.9, 2.1)	16m 42s
	5h 39m 58s

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