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
```

store/sqlstore/store.go

@@ -30,12 +30,12 @@ import (
 	"github.com/mattermost/gorp"
 	"github.com/pkg/errors"
 
-	"github.com/mattermost/mattermost-server/v5/db/migrations"
-	"github.com/mattermost/mattermost-server/v5/einterfaces"
-	"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/store"
+	"github.com/mattermost/mattermost-server/v6/db/migrations"
+	"github.com/mattermost/mattermost-server/v6/einterfaces"
+	"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/store"
 )
 
 type migrationDirection string
@@ -139,7 +139,7 @@ type SqlStoreStores struct {
 
 type SqlStore struct {
 	// rrCounter and srCounter should be kept first.
-	// See https://github.com/mattermost/mattermost-server/v5/pull/7281
+	// See https://github.com/mattermost/mattermost-server/v6/pull/7281
 	rrCounter         int64
 	srCounter         int64
 	master            *gorp.DbMap