s/masterminds/mattermost for squirrel (#20162)

Our fork of https://github.com/Masterminds/squirrel at https://github.com/mattermost/squirrel (previously https://github.com/lieut-data/squirrel) isn't compatible with the upstream project, and causes issues for repositories importing mattermost-server and using squirrel.

Sidestep this by adopting the `mattermost/squirrel` fork explicitly.  (Downstream repos can either switch, or keep using the upstream version from `Masterminds`.)

store/sqlstore/command_store.go

@@ -7,7 +7,7 @@ import (
 	"database/sql"
 	"fmt"
 
-	sq "github.com/Masterminds/squirrel"
+	sq "github.com/mattermost/squirrel"
 	"github.com/pkg/errors"
 
 	"github.com/mattermost/mattermost-server/v6/model"