lnrpc: sort `Invoice.HTLCs` based on `HtlcIndex` #16827

Job	Run time
RPC and mobile compilation check	2m 0s
Sqlc check	2m 41s
check commits	8m 40s
cross compilation	31m 22s
lint code	4m 9s
sample configuration check	47s
run unit tests (btcd unit-cover)	19m 16s
run unit tests (unit tags="kvdb_etcd")	20m 47s
check pinned dependencies (google.golang.org/grpc v1.59.0)	3s
run unit tests (unit tags="kvdb_postgres")	19m 45s
run unit tests (unit tags="kvdb_sqlite")	19m 16s
check pinned dependencies (github.com/golang/protobuf v1.5.3)	2s
run unit tests (btcd unit-race)	40m 23s
run unit tests (unit-module)	1m 15s
run macOS itest	50m 6s
run ubuntu itests (btcd, backend=btcd cover=1)	31m 32s
run ubuntu itests (bitcoind, backend=bitcoind cover=1)	39m 48s
run windows itest	6m 19s
run ubuntu itests (bitcoind-notxindex, backend="bitcoind notxindex")	40m 12s
run ubuntu itests (bitcoind-rpcpolling, backend="bitcoind rpcpolling" cover=1)	39m 44s
check release notes updated	3s
run ubuntu itests (bitcoind-etcd, backend=bitcoind dbbackend=etcd)	52m 36s
run ubuntu itests (bitcoind-postgres, backend=bitcoind dbbackend=postgres)	1h 9m 37s
run ubuntu itests (bitcoind-sqlite, backend=bitcoind dbbackend=sqlite)	42m 7s
run ubuntu itests (bitcoind-postgres-nativesql, backend=bitcoind dbbackend=postgres nativesql=true)	1h 12m 11s
run ubuntu itests (bitcoind-sqlite-nativesql, backend=bitcoind dbbackend=sqlite nativesql=true)	42m 30s
run ubuntu itests (neutrino, backend=neutrino cover=1)	38m 9s
finish	3s
	11h 35m 23s

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