Update build sdk images to 7.0

To support net6

Author: startnow65

.ci/tasks/build.yaml

@@ -4,7 +4,7 @@ image_resource:
   type: docker-image
   source:
     repository: mcr.microsoft.com/dotnet/sdk
-    tag: 5.0.103-alpine3.12-amd64
+    tag: 7.0-alpine3.15-amd64
 inputs:
 - name: source
 params:

.ci/tasks/publish-nugget.yaml

@@ -4,7 +4,7 @@ image_resource:
   type: docker-image
   source:
     repository: mcr.microsoft.com/dotnet/sdk
-    tag: 5.0.103-alpine3.12-amd64
+    tag: 7.0-alpine3.15-amd64
 inputs:
 - name: source
 - name: version