bump code.cloudfoundry.org/clock to v1.15.0 #3744

Job	Run time
Basic Checks	2m 59s
Unit Tests	25m 3s
Integration Tests (raft)	12m 41s
Integration Tests (pvtdata)	11m 41s
Integration Tests (pvtdatapurge e2e)	11m 11s
Integration Tests (ledger)	12m 31s
Integration Tests (lifecycle)	6m 29s
Integration Tests (smartbft)	19m 59s
Integration Tests (discovery gossip devmode pluggable)	6m 23s
Integration Tests (gateway idemix pkcs11 configtx configtxlator)	17m 10s
Integration Tests (sbe nwo msp)	7m 43s
	2h 13m 50s

Provide feedback