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
```

web/context.go

@@ -9,13 +9,13 @@ import (
 	"regexp"
 	"strings"
 
-	"github.com/mattermost/mattermost-server/v5/app"
-	"github.com/mattermost/mattermost-server/v5/app/request"
-	"github.com/mattermost/mattermost-server/v5/audit"
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/shared/i18n"
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
-	"github.com/mattermost/mattermost-server/v5/utils"
+	"github.com/mattermost/mattermost-server/v6/app"
+	"github.com/mattermost/mattermost-server/v6/app/request"
+	"github.com/mattermost/mattermost-server/v6/audit"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/shared/i18n"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/utils"
 )
 
 type Context struct {