Update vendor slack-go/slack (#1068)

vendor/github.com/slack-go/slack/block_unknown.go

@@ -0,0 +1,13 @@
+package slack
+
+// UnknownBlock represents a block type that is not yet known. This block type exists to prevent Slack from introducing
+// new and unknown block types that break this library.
+type UnknownBlock struct {
+	Type    MessageBlockType `json:"type"`
+	BlockID string           `json:"block_id,omitempty"`
+}
+
+// BlockType returns the type of the block
+func (b UnknownBlock) BlockType() MessageBlockType {
+	return b.Type
+}