Update versions.json to include v6.12-rc2 #135

Job	Run time
Read config	5s
Kernel (5.10.226) / build-and-push	11m 58s
Kernel (5.15.167) / build-and-push	12m 59s
Kernel (4.19.322) / build-and-push	9m 28s
Kernel (5.4.284) / build-and-push	10m 9s
Kernel (6.1.112) / build-and-push	13m 59s
Kernel (6.10.13) / build-and-push	15m 30s
Kernel (6.12-rc2, mainline) / build-and-push	36m 47s
Kernel (6.11.1, stable) / build-and-push	36m 4s
Kernel (6.6.41, longterm) / build-and-push	33m 4s
Results	1s
	3h 0m 4s

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