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/driver/conn.go

@@ -7,7 +7,7 @@ import (
 	"context"
 	"database/sql/driver"
 
-	"github.com/mattermost/mattermost-server/v5/plugin"
+	"github.com/mattermost/mattermost-server/v6/plugin"
 )
 
 // Conn is a DB driver conn implementation

shared/driver/driver.go

@@ -12,7 +12,7 @@ import (
 	"context"
 	"database/sql/driver"
 
-	"github.com/mattermost/mattermost-server/v5/plugin"
+	"github.com/mattermost/mattermost-server/v6/plugin"
 )
 
 var (

shared/driver/objects.go

@@ -7,7 +7,7 @@ import (
 	"context"
 	"database/sql/driver"
 
-	"github.com/mattermost/mattermost-server/v5/plugin"
+	"github.com/mattermost/mattermost-server/v6/plugin"
 )
 
 type wrapperTx struct {