Upgrade to gopkg.in/yaml.v3

cmd/acmeserver/acmeserver.go

config.go

go.mod

go.sum

vendor/gopkg.in/yaml.v2/.travis.yml

vendor/gopkg.in/yaml.v2/LICENSE

vendor/gopkg.in/yaml.v2/go.mod

vendor/gopkg.in/yaml.v3/LICENSE

vendor/gopkg.in/yaml.v3/NOTICE

vendor/gopkg.in/yaml.v3/README.md

vendor/gopkg.in/yaml.v3/apic.go

vendor/gopkg.in/yaml.v3/decode.go

vendor/gopkg.in/yaml.v3/emitterc.go

vendor/gopkg.in/yaml.v3/encode.go

vendor/gopkg.in/yaml.v3/go.mod

vendor/gopkg.in/yaml.v3/parserc.go

vendor/gopkg.in/yaml.v3/readerc.go

vendor/gopkg.in/yaml.v3/resolve.go

vendor/gopkg.in/yaml.v3/scannerc.go

vendor/gopkg.in/yaml.v3/sorter.go