Update to 2021.2

Sync to https://github.com/ostreedev/ostree/releases/tag/v2021.2

rust-bindings/rust/conf/ostree.toml

@@ -19,6 +19,7 @@ generate = [
     "OSTree.AsyncProgress",
     "OSTree.BootconfigParser",
     "OSTree.ChecksumFlags",
+    "OSTree.ContentWriter",
     "OSTree.CommitSizesEntry",
     "OSTree.Deployment",
     "OSTree.DeploymentUnlockedState",
@@ -71,6 +72,7 @@ manual = [
     "Gio.FileQueryInfoFlags",
     "Gio.FilterInputStream",
     "Gio.InputStream",
+    "Gio.OutputStream",
     "Gio.VolumeMonitor",
     "GLib.Bytes",
     "GLib.Checksum",