`TransactionPool` API uses `async_trait` #14217

Job	Run time
preflight / preflight	42s
preflight / ci-versions	59s
check-runtime-migration (westend)	6m 19s
check-runtime-migration (asset-hub-westend)	4m 51s
check-runtime-migration (bridge-hub-westend)	4m 15s
check-runtime-migration (collectives-westend)	4m 12s
check-runtime-migration (coretime-westend)	4m 19s
All runtime migrations passed	1s
	25m 38s

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