chore(deps): upgrade zstd seekable package (#37988)

Upgrade `github.com/SaveTheRbtz/zstd-seekable-format-go/pkg` from
`v0.8.3` to `v0.10.0`:

https://github.com/SaveTheRbtz/zstd-seekable-format-go/releases/tag/pkg%2Fv0.10.0

This keeps Gitea's seekable zstd wrapper on the stable v0.10 API while
preserving the existing public `modules/zstd` API.

API migration:
- update `SeekableWriter` and `SeekableReader` internals for the
concrete `*seekable.Writer` and `*seekable.Reader` types introduced by
SaveTheRbtz/zstd-seekable-format-go#264
- update generated dependency metadata after `go mod tidy` removed the
now-unused `github.com/google/btree` transitive dependency
- no Gitea call sites needed changes because `modules/zstd` still
exposes the same constructors and interfaces

Validation:
- `go test ./modules/zstd`
- `make --always-make checks-backend`

---------

Co-authored-by: Giteabot <teabot@gitea.io>

modules/zstd/zstd.go

@@ -60,7 +60,7 @@ func (r *Reader) Close() error {
 type SeekableWriter struct {
 	buf []byte
 	n   int
-	w   seekable.Writer
+	w   *seekable.Writer
 }
 
 var _ io.WriteCloser = (*SeekableWriter)(nil)
@@ -114,7 +114,7 @@ func (w *SeekableWriter) Close() error {
 }
 
 type SeekableReader struct {
-	r seekable.Reader
+	r *seekable.Reader
 	c func() error
 }