Support legacy _links LFS batch responses (#31513)

Support legacy _links LFS batch response.

Fixes #31512.

This is backwards-compatible change to the LFS client so that, upon
mirroring from an upstream which has a batch api, it can download
objects whether the responses contain the `_links` field or its
successor the `actions` field. When Gitea must fallback to the legacy
`_links` field a logline is emitted at INFO level which looks like this:
```
...s/lfs/http_client.go:188:performOperation() [I] <LFSPointer ee95d0a27ccdfc7c12516d4f80dcf144a5eaf10d0461d282a7206390635cdbee:160> is using a deprecated batch schema response!
```

I've only run `test-backend` with this code, but added a new test to
cover this case. Additionally I have a fork with this change deployed
which I've confirmed syncs LFS from Gitea<-Artifactory (which has legacy
`_links`) as well as from Gitea<-Gitea (which has the modern `actions`).

Signed-off-by: Royce Remer <royceremer@gmail.com>

modules/lfs/http_client.go

@@ -180,6 +180,10 @@ func (c *HTTPClient) performOperation(ctx context.Context, objects []Pointer, dc
 			}
 		} else {
 			link, ok := object.Actions["download"]
+			if !ok {
+				// no actions block in response, try legacy response schema
+				link, ok = object.Links["download"]
+			}
 			if !ok {
 				log.Debug("%+v", object)
 				return errors.New("missing action 'download'")