v8.0 module release (#22975)

https://mattermost.atlassian.net/browse/MM-52079

```release-note
We upgrade the module version to 8.0. The new module path is github.com/mattermost-server/server/v8.
```


Co-authored-by: Doug Lauder <wiggin77@warpmail.net>

server/channels/store/layer_generators/opentracing_layer.go.tmpl

@@ -10,9 +10,9 @@ import (
 	"context"
 	"time"
 
-	"github.com/mattermost/mattermost-server/v6/model"
-	"github.com/mattermost/mattermost-server/v6/server/platform/services/tracing"
-	"github.com/mattermost/mattermost-server/v6/server/channels/store"
+	"github.com/mattermost/mattermost-server/server/v8/model"
+	"github.com/mattermost/mattermost-server/server/v8/platform/services/tracing"
+	"github.com/mattermost/mattermost-server/server/v8/channels/store"
 	"github.com/opentracing/opentracing-go/ext"
 	spanlog "github.com/opentracing/opentracing-go/log"
 )

server/channels/store/layer_generators/retry_layer.go.tmpl

@@ -12,8 +12,8 @@ import (
 	"time"
 
 	"github.com/lib/pq"
-	"github.com/mattermost/mattermost-server/v6/model"
-    "github.com/mattermost/mattermost-server/v6/server/channels/store"
+	"github.com/mattermost/mattermost-server/server/v8/model"
+    "github.com/mattermost/mattermost-server/server/v8/channels/store"
     "github.com/pkg/errors"
 	"github.com/go-sql-driver/mysql"
 )

server/channels/store/layer_generators/timer_layer.go.tmpl

@@ -10,9 +10,9 @@ import (
 	"context"
     "time"
 
-    "github.com/mattermost/mattermost-server/v6/server/channels/einterfaces"
-	"github.com/mattermost/mattermost-server/v6/model"
-	"github.com/mattermost/mattermost-server/v6/server/channels/store"
+    "github.com/mattermost/mattermost-server/server/v8/channels/einterfaces"
+	"github.com/mattermost/mattermost-server/server/v8/model"
+	"github.com/mattermost/mattermost-server/server/v8/channels/store"
 )
 
 type {{.Name}} struct {