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/store.go

@@ -16,8 +16,8 @@ import (
 	"sync/atomic"
 	"time"
 
-	sq "github.com/Masterminds/squirrel"
 	"github.com/mattermost/morph"
+	sq "github.com/mattermost/squirrel"
 
 	"github.com/mattermost/morph/drivers"
 	ms "github.com/mattermost/morph/drivers/mysql"