Fix DownloadFunc when migrating releases (#27887)

[GiteaBot]

Backport #27887 by @Zettat123

We should not use asset.ID in DownloadFunc because DownloadFunc is a closure.

gitea/services/migrations/gitea_downloader.go

Lines 284 to 295 in 1bf5527

	for _, asset := range rel.Attachments {
	size := int(asset.Size)
	dlCount := int(asset.DownloadCount)
	r.Assets = append(r.Assets, &base.ReleaseAsset{
	ID: asset.ID,
	Name: asset.Name,
	Size: &size,
	DownloadCount: &dlCount,
	Created: asset.Created,
	DownloadURL: &asset.DownloadURL,
	DownloadFunc: func() (io.ReadCloser, error) {
	asset, _, err := g.client.GetReleaseAttachment(g.repoOwner, g.repoName, rel.ID, asset.ID)

A similar bug when migrating from GitHub has been fixed in #14703. This PR fixes the bug when migrating from Gitea and GitLab.