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/v115.go

@@ -16,7 +16,6 @@ import (
 	"gitea.dev/modules/container"
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/setting"
-	"gitea.dev/modules/util"
 )
 
 func RenameExistingUserAvatarName(x base.EngineMigration) error {
@@ -110,8 +109,8 @@ func RenameExistingUserAvatarName(x base.EngineMigration) error {
 	log.Info("Deleting %d old avatars ...", deleteCount)
 	i := 0
 	for file := range deleteList {
-		if err := util.Remove(file); err != nil {
-			log.Warn("util.Remove: %v", err)
+		if err := os.Remove(file); err != nil {
+			log.Warn("Failed to remove avatar %s: %v", file, err)
 		}
 		i++
 		select {