MM-31356: Add a minimum required version check for Postgres (#16597)

* MM-31356: Add a minimum required version check for Postgres

To keep conformance with our failing fast and obvious philosophy,
we add a check to prevent Mattermost server from starting
if the postgres version is below 10.0.

This gives customers a chance to upgrade their database before upgrading
their Mattermost version, than to run into weird compatibility issues
after they have finished the upgrade.

https://mattermost.atlassian.net/browse/MM-31356

```release-note
NONE
```

* fix lint errors

* Use a function to pretty-print version string

* rectify comment

app/server.go

@@ -16,6 +16,7 @@ import (
 	"path"
 	"path/filepath"
 	"runtime"
+	"strconv"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -319,6 +320,19 @@ func NewServer(options ...Option) (*Server, error) {
 	if s.newStore == nil {
 		s.newStore = func() (store.Store, error) {
 			s.sqlStore = sqlstore.New(s.Config().SqlSettings, s.Metrics)
+			if s.sqlStore.DriverName() == model.DATABASE_DRIVER_POSTGRES {
+				ver, err2 := s.sqlStore.GetDbVersion(true)
+				if err2 != nil {
+					return nil, errors.Wrap(err2, "cannot get DB version")
+				}
+				intVer, err2 := strconv.Atoi(ver)
+				if err2 != nil {
+					return nil, errors.Wrap(err2, "cannot parse DB version")
+				}
+				if intVer < sqlstore.MINIMUM_REQUIRED_POSTGRES_VERSION {
+					return nil, fmt.Errorf("minimum required postgres version is %s; found %s", sqlstore.VersionString(sqlstore.MINIMUM_REQUIRED_POSTGRES_VERSION), sqlstore.VersionString(intVer))
+				}
+			}
 
 			lcl, err2 := localcachelayer.NewLocalCacheLayer(
 				retrylayer.New(s.sqlStore),