Refactor webhook (#31587)

A more complete fix for #31588

1. Make "generic" code more readable
2. Clarify HTML or Markdown for the payload content

services/webhook/matrix.go

@@ -29,10 +29,10 @@ func newMatrixRequest(_ context.Context, w *webhook_model.Webhook, t *webhook_mo
 	if err := json.Unmarshal([]byte(w.Meta), meta); err != nil {
 		return nil, nil, fmt.Errorf("GetMatrixPayload meta json: %w", err)
 	}
-	mc := matrixConvertor{
+	var pc payloadConvertor[MatrixPayload] = matrixConvertor{
 		MsgType: messageTypeText[meta.MessageType],
 	}
-	payload, err := newPayload(mc, []byte(t.PayloadContent), t.EventType)
+	payload, err := newPayload(pc, []byte(t.PayloadContent), t.EventType)
 	if err != nil {
 		return nil, nil, err
 	}
@@ -87,8 +87,6 @@ type MatrixPayload struct {
 	Commits       []*api.PayloadCommit `json:"io.gitea.commits,omitempty"`
 }
 
-var _ payloadConvertor[MatrixPayload] = matrixConvertor{}
-
 type matrixConvertor struct {
 	MsgType string
 }