Prevent timer leaks in Workerpool and others (#11333)

There is a potential memory leak in `Workerpool` due to the intricacies of
`time.Timer` stopping.

Whenever a `time.Timer` is `Stop`ped its channel must be cleared using a
`select` if the result of the `Stop()` is `false`.

Unfortunately in `Workerpool` these were checked the wrong way round.

However, there were a few other places that were not being checked.

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/migrations/github.go

@@ -16,6 +16,7 @@ import (
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/migrations/base"
 	"code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/modules/util"
 
 	"github.com/google/go-github/v24/github"
 	"golang.org/x/oauth2"
@@ -121,7 +122,7 @@ func (g *GithubDownloaderV3) sleep() {
 		timer := time.NewTimer(time.Until(g.rate.Reset.Time))
 		select {
 		case <-g.ctx.Done():
-			timer.Stop()
+			util.StopTimer(timer)
 			return
 		case <-timer.C:
 		}