MM-36271: Bump major version to 6.0 (#17973)

https://mattermost.atlassian.net/browse/MM-36271

```release-note
We bump the major version to 6.0
```

shared/mlog/global_test.go

@@ -13,7 +13,7 @@ import (
 
 	"github.com/stretchr/testify/require"
 
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 func TestLoggingBeforeInitialized(t *testing.T) {

shared/mlog/human/entry.go

@@ -8,7 +8,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 type LogEntry struct {

shared/mlog/human/parser.go

@@ -12,7 +12,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 func ParseLogMessage(msg string) LogEntry {

shared/mlog/log_test.go

@@ -10,7 +10,7 @@ import (
 
 	"github.com/stretchr/testify/require"
 
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 // Test race condition when shutting down advanced logging. This test must run with the -race flag in order to verify