Support migrating inconsistent bucket policies (#5855)

Previously we used allow bucket policies without
`Version` field to be set to any given value, but
this behavior is inconsistent with AWS S3.

PR #5790 addressed this by making bucket policies
stricter and cleaner, but this causes a breaking
change causing any existing policies perhaps without
`Version` field or the field to be empty to fail upon
server startup.

This PR brings a code to migrate under these scenarios
as a one time operation.

cmd/bucket-policy-handlers.go

@@ -79,6 +79,12 @@ func (api objectAPIHandlers) PutBucketPolicyHandler(w http.ResponseWriter, r *ht
 		return
 	}
 
+	// Version in policy must not be empty
+	if bucketPolicy.Version == "" {
+		writeErrorResponse(w, ErrMalformedPolicy, r.URL)
+		return
+	}
+
 	if err = objAPI.SetBucketPolicy(ctx, bucket, bucketPolicy); err != nil {
 		writeErrorResponse(w, toAPIErrorCode(err), r.URL)
 		return