Migrate `pallet-alliance` to umbrella crate #15226

Job	Run time
preflight / preflight	1m 21s
preflight / ci-versions	1m 1s
check-runtime-migration (westend)	7m 48s
check-runtime-migration (asset-hub-westend)	5m 34s
check-runtime-migration (bridge-hub-westend)	4m 38s
check-runtime-migration (collectives-westend)	4m 43s
check-runtime-migration (coretime-westend)	5m 14s
All runtime migrations passed	1s
	30m 20s

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