MM-47465: Fix flaky TestDeleteChannel (#21686)

Creating the post in a separate goroutine would create a race condition
because that method also calls GetChannel. This can cause a bug
if the cache was wiped before the the other goroutine would get
a chance to update the cache. And if that happens, then
it would populate the cache with the old value.

Pseudo-code

```
func deleteChannel() {
    go func() {
        getFromCache()
    }()
    updateDB()
    wipeCache() // a
}

func getFromCache() {
    err := checkCache()
    if err == ErrNotFound {
        getFromDB() // x
        updateCache() // y
    }
}

func Test() {
    deleteChannel()
    getFromCache()
}
```

If the sequence of events happen like
- x
- a
- y

Then the getFromCache() call later will get the wrong
value from cache.

The fix is to make the call synchronous.

https://mattermost.atlassian.net/browse/MM-47465

Special thanks to @noxer for finding the root cause.

```release-note
NONE
```

api4/channel_test.go

@@ -1779,7 +1779,6 @@ func TestSearchGroupChannels(t *testing.T) {
 }
 
 func TestDeleteChannel(t *testing.T) {
-	t.Skip("MM-47465")
 	th := Setup(t).InitBasic()
 	defer th.TearDown()
 	c := th.Client

app/channel.go

@@ -1420,13 +1420,10 @@ func (a *App) DeleteChannel(c request.CTX, channel *model.Channel, userID string
 			c.Logger().Warn("Failed to post archive message", mlog.Err(err))
 		}
 	} else {
-		a.Srv().Go(func() {
-			systemBot, err := a.GetSystemBot()
-			if err != nil {
-				c.Logger().Error("Failed to post archive message", mlog.Err(err))
-				return
-			}
-
+		systemBot, err := a.GetSystemBot()
+		if err != nil {
+			c.Logger().Warn("Failed to post archive message", mlog.Err(err))
+		} else {
 			post := &model.Post{
 				ChannelId: channel.Id,
 				Message:   fmt.Sprintf(i18n.T("api.channel.delete_channel.archived"), systemBot.Username),
@@ -1438,9 +1435,9 @@ func (a *App) DeleteChannel(c request.CTX, channel *model.Channel, userID string
 			}
 
 			if _, err := a.CreatePost(c, post, channel, false, true); err != nil {
-				c.Logger().Error("Failed to post archive message", mlog.Err(err))
+				c.Logger().Warn("Failed to post archive message", mlog.Err(err))
 			}
-		})
+		}
 	}
 
 	now := model.GetMillis()