refactor: retry file remove/rename when a file is busy and clean up os detection (#38588)

Rename functions "util.Remove" (remove.go) to "util.RemoveWithRetry"
(file_retry.go) and add comments to clarify their behaviors, also add
tests.

Refactor callers: when no concurrent access (cmd cli, migration, app
init, test), use "os.Xxx" directly.

More details are in `modules/util/file_retry.go`

By the way, clean up OS (windows) detection, make FileURLToPath test
always run

modelmigration/v1_11/v112.go

@@ -4,12 +4,12 @@
 package v1_11
 
 import (
+	"os"
 	"path/filepath"
 
 	"gitea.dev/modelmigration/base"
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/setting"
-	"gitea.dev/modules/util"
 
 	"xorm.io/builder"
 )
@@ -30,7 +30,7 @@ func RemoveAttachmentMissedRepo(x base.EngineMigration) error {
 
 		for i := 0; i < len(attachments); i++ {
 			uuid := attachments[i].UUID
-			if err = util.RemoveAll(filepath.Join(setting.Attachment.Storage.Path, uuid[0:1], uuid[1:2], uuid)); err != nil {
+			if err = os.RemoveAll(filepath.Join(setting.Attachment.Storage.Path, uuid[0:1], uuid[1:2], uuid)); err != nil {
 				log.Warn("Unable to remove attachment file by UUID %s: %v", uuid, err)
 			}
 		}