Remove global variables and fix racy test (#18733)

https://community.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4?c=65b44de0-1a9c-4643-84c1-5d803593f314

This PR improves on two things at once:
- It fixes the race condition by removing global variables.
- But instead of moving them to Server, it moves them inside Channels,
thereby making one more step towards the multi-product architecture.

```release-note
NONE
```

app/channels.go

@@ -4,6 +4,7 @@
 package app
 
 import (
+	"github.com/mattermost/mattermost-server/v6/model"
 	"github.com/mattermost/mattermost-server/v6/services/imageproxy"
 )
 
@@ -12,6 +13,13 @@ type Channels struct {
 	srv *Server
 
 	imageProxy *imageproxy.ImageProxy
+
+	// cached counts that are used during notice condition validation
+	cachedPostCount   int64
+	cachedUserCount   int64
+	cachedDBMSVersion string
+	// previously fetched notices
+	cachedNotices model.ProductNotices
 }
 
 func init() {