Bump sigstore/sigstore to support KMS plugins (#4073)

This allows users to implement their own KMS providers as separate
programs. See the sigstore/sigstore repo for more information.

Signed-off-by: Hayden B <[email protected]>

Author: haydentherapper

go.mod

@@ -34,7 +34,7 @@ require (
 	github.com/sigstore/fulcio v1.6.6
 	github.com/sigstore/protobuf-specs v0.4.0
 	github.com/sigstore/rekor v1.3.9
-	github.com/sigstore/sigstore v1.8.12
+	github.com/sigstore/sigstore v1.8.15
 	github.com/sigstore/sigstore-go v0.7.0
 	github.com/sigstore/sigstore/pkg/signature/kms/aws v1.8.12
 	github.com/sigstore/sigstore/pkg/signature/kms/azure v1.8.12

go.sum

@@ -617,8 +617,8 @@ github.com/sigstore/protobuf-specs v0.4.0 h1:yoZbdh0kZYKOSiVbYyA8J3f2wLh5aUk2SQB
 github.com/sigstore/protobuf-specs v0.4.0/go.mod h1:FKW5NYhnnFQ/Vb9RKtQk91iYd0MKJ9AxyqInEwU6+OI=
 github.com/sigstore/rekor v1.3.9 h1:sUjRpKVh/hhgqGMs0t+TubgYsksArZ6poLEC3MsGAzU=
 github.com/sigstore/rekor v1.3.9/go.mod h1:xThNUhm6eNEmkJ/SiU/FVU7pLY2f380fSDZFsdDWlcM=
-github.com/sigstore/sigstore v1.8.12 h1:S8xMVZbE2z9ZBuQUEG737pxdLjnbOIcFi5v9UFfkJFc=
-github.com/sigstore/sigstore v1.8.12/go.mod h1:+PYQAa8rfw0QdPpBcT+Gl3egKD9c+TUgAlF12H3Nmjo=
+github.com/sigstore/sigstore v1.8.15 h1:9HHnZmxjPQSTPXTCZc25HDxxSTWwsGMh/ZhWZZ39maU=
+github.com/sigstore/sigstore v1.8.15/go.mod h1:+Wa5mrG6A+Gss516YC9owy10q3IazqIRe0y1EoQRHHM=
 github.com/sigstore/sigstore-go v0.7.0 h1:bIGPc2IbnbxnzlqQcKlh1o96bxVJ4yRElpP1gHrOH48=
 github.com/sigstore/sigstore-go v0.7.0/go.mod h1:4RrCK+i+jhx7lyOG2Vgef0/kFLbKlDI1hrioUYvkxxA=
 github.com/sigstore/sigstore/pkg/signature/kms/aws v1.8.12 h1:EC3UmIaa7nV9sCgSpVevmvgvTYTkMqyrRbj5ojPp7tE=