Fix encoding for Matrix Webhooks (#37190)

`url.PathEscape` unnecessarily encodes ! to %21, causing Matrix
homeservers to reject the request with 401. Replace %21 back to ! after
escaping.

Fixes #36012

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/repo/setting/webhook_test.go

@@ -0,0 +1,15 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package setting
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestWebhookMatrix(t *testing.T) {
+	assert.Equal(t, "!roomid:domain", matrixRoomIDEncode("!roomid:domain"))
+	assert.Equal(t, "!room%23id:domain", matrixRoomIDEncode("!room#id:domain")) // maybe it should never really happen in real world
+}