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/platform/services/docextractor/combine.go

@@ -6,7 +6,7 @@ package docextractor
 import (
 	"io"
 
-	"github.com/mattermost/mattermost-server/v6/server/platform/shared/mlog"
+	"github.com/mattermost/mattermost-server/server/v8/platform/shared/mlog"
 )
 
 type combineExtractor struct {

server/platform/services/docextractor/docextractor_test.go

@@ -13,7 +13,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"github.com/mattermost/mattermost-server/v6/server/channels/utils/testutils"
+	"github.com/mattermost/mattermost-server/server/v8/channels/utils/testutils"
 )
 
 func TestExtract(t *testing.T) {

server/platform/services/docextractor/pdf_test.go

@@ -9,7 +9,7 @@ import (
 
 	"github.com/stretchr/testify/require"
 
-	"github.com/mattermost/mattermost-server/v6/server/channels/utils/testutils"
+	"github.com/mattermost/mattermost-server/server/v8/channels/utils/testutils"
 )
 
 func TestPdfEmptyFile(t *testing.T) {