go.mod: bump go-immutable-radix to v1.3.1

Fixes errors in tooling that relies on the existing `go.mod` file:

```
pkg/darwin_arm64/github.com/armon/go-metrics/metrics.go, line 8, column 2: could not import github.com/hashicorp/go-immutable-radix (open : no such file or directory)
pkg/darwin_arm64/github.com/armon/go-metrics/metrics.go, line 166, column 13: undefined: iradix
pkg/darwin_arm64/github.com/armon/go-metrics/start.go, line 37, column 17: undefined: iradix
```

Author: sean-

go.mod

@@ -7,7 +7,7 @@ require (
 	github.com/circonus-labs/circonus-gometrics v2.3.1+incompatible
 	github.com/circonus-labs/circonusllhist v0.1.3 // indirect
 	github.com/golang/protobuf v1.3.2
-	github.com/hashicorp/go-immutable-radix v1.0.0
+	github.com/hashicorp/go-immutable-radix v1.3.1
 	github.com/hashicorp/go-retryablehttp v0.5.3 // indirect
 	github.com/pascaldekloe/goe v0.1.0
 	github.com/prometheus/client_golang v1.4.0
@@ -17,4 +17,4 @@ require (
 )
 
 // Introduced undocumented breaking change to metrics sink interface
-retract v0.3.11
+retract v0.3.11

go.sum

@@ -38,6 +38,8 @@ github.com/hashicorp/go-cleanhttp v0.5.0 h1:wvCrVc9TjDls6+YGAF2hAifE1E5U1+b4tH6K
 github.com/hashicorp/go-cleanhttp v0.5.0/go.mod h1:JpRdi6/HCYpAwUzNwuwqhbovhLtngrth3wmdIIUrZ80=
 github.com/hashicorp/go-immutable-radix v1.0.0 h1:AKDB1HM5PWEA7i4nhcpwOrO2byshxBjXVn/J/3+z5/0=
 github.com/hashicorp/go-immutable-radix v1.0.0/go.mod h1:0y9vanUI8NX6FsYoO3zeMjhV/C5i9g4Q3DwcSNZ4P60=
+github.com/hashicorp/go-immutable-radix v1.3.1 h1:DKHmCUm2hRBK510BaiZlwvpD40f8bJFeZnpfm2KLowc=
+github.com/hashicorp/go-immutable-radix v1.3.1/go.mod h1:0y9vanUI8NX6FsYoO3zeMjhV/C5i9g4Q3DwcSNZ4P60=
 github.com/hashicorp/go-retryablehttp v0.5.3 h1:QlWt0KvWT0lq8MFppF9tsJGF+ynG7ztc2KIPhzRGk7s=
 github.com/hashicorp/go-retryablehttp v0.5.3/go.mod h1:9B5zBasrRhHXnJnui7y6sL7es7NDiJgTc6Er0maI1Xs=
 github.com/hashicorp/go-uuid v1.0.0 h1:RS8zrF7PhGwyNPOtxSClXXj9HA8feRnJzgnI1RJCSnM=