Update refs to :soroban-dev to ref :future in readme #600

Job	Run time
amd64 / build-stellar-core	24s
amd64 / build-stellar-friendbot	59s
amd64 / build-stellar-horizon	2m 39s
amd64 / build-rs-stellar-xdr	2m 7s
amd64 / build-stellar-soroban-rpc	1m 44s
arm64 / build-stellar-core	20s
arm64 / build-stellar-horizon	8m 52s
arm64 / build-stellar-friendbot	4m 45s
arm64 / build-rs-stellar-xdr	2m 54s
arm64 / build-stellar-soroban-rpc	2m 0s
amd64 / build	5m 14s
amd64 / test (pubnet)	1m 32s
amd64 / test (local)	1m 45s
amd64 / test (local, --enable-soroban-rpc)	1m 55s
amd64 / push-pr	56s
arm64 / build	7m 30s
amd64 / push-release	0s
arm64 / test (pubnet)	1m 25s
arm64 / test (local)	1m 46s
arm64 / push-pr	44s
arm64 / test (local, --enable-soroban-rpc)	3m 7s
arm64 / push-release	0s
manifest / push	5s
complete	1s
	52m 44s

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