multi: remove deprecated io/ioutil

[argentpapa]

Change Description

ioutil.ReadAll is deprecated: As of Go 1.16, this function simply calls io.ReadAll

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Steps for reviewers to follow to test the change.

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.
• Bug fixes contain tests triggering the bug to prevent regressions.

Code Style and Documentation

• The change is not insubstantial. Typo fixes are not accepted to fight bot spam.
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[guggero]

Thanks, LGTM 🎉

[coveralls]

Pull Request Test Coverage Report for Build 13746269182

Details

• 0 of 3 (0.0%) changed or added relevant lines in 3 files are covered.
• 51 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.09%) to 55.238%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/btcctl/config.go	0	1	0.0%
rpcclient/infrastructure.go	0	1	0.0%
rpcserver.go	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
peer/peer.go	15	73.57%
wire/msgaddrv2.go	16	51.72%
wire/netaddressv2.go	20	74.45%

Totals
Change from base Build 13719035689:	-0.09%
Covered Lines:	29900
Relevant Lines:	54129

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💛 - Coveralls