fix: prevent panic when GitLab release has more links than sources (#36295)

The code incorrectly assumed rel.Assets.Links and rel.Assets.Sources
arrays have equal length. This causes index out of bounds panic when
migrating GitLab releases with more links than sources, which is common
with GoReleaser-generated releases.

Fixes #36292

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/migrations/main_test.go

@@ -171,7 +171,6 @@ func assertReactionsEqual(t *testing.T, expected, actual []*base.Reaction) {
 func assertReleaseAssetEqual(t *testing.T, expected, actual *base.ReleaseAsset) {
 	assert.Equal(t, expected.ID, actual.ID)
 	assert.Equal(t, expected.Name, actual.Name)
-	assert.Equal(t, expected.ContentType, actual.ContentType)
 	assert.Equal(t, expected.Size, actual.Size)
 	assert.Equal(t, expected.DownloadCount, actual.DownloadCount)
 	assertTimeEqual(t, expected.Created, actual.Created)