Update server hook documentation to reflect current defaults

Go update hook feature flag has been removed and is always on.
Pre-receive hook is also switched on by default although the feature
flag is still present.

doc/administration/server_hooks.md

@@ -158,18 +158,18 @@ them as they can change.
 
 ## Transition to Go
 
-> Introduced in GitLab 13.2 using feature flags.
+> - Introduced in GitLab 13.2 using feature flags.
+> - In GitLab 13.4, `update` Ruby [implementation removed](https://gitlab.com/gitlab-org/gitaly/-/merge_requests/2501).
+> - In GitLab 13.4, `post-receive` Go implementation [made default](https://gitlab.com/gitlab-org/gitaly/-/merge_requests/2502).
 
 The following server hooks have been re-implemented in Go:
 
 - `pre-receive`, with the Go implementation used by default. To use the Ruby implementation instead,
   [disable](feature_flags.md#enable-or-disable-the-feature) the `:gitaly_go_preceive_hook` feature
   flag.
-- `update`, with the Go implementation used by default. To use the Ruby implementation instead,
-  [disable](feature_flags.md#enable-or-disable-the-feature) the `:gitaly_go_update_hook` feature
-  flag.
-- `post-receive`, however the Ruby implementation is used by default. To use the Go implementation
-  instead, [enable](feature_flags.md#enable-or-disable-the-feature) the
+- `update`, with Go implementation always used. No Ruby implementation is available.
+- `post-receive`, with the Go implementation used by default. To use the Ruby implementation
+  instead, [disable](feature_flags.md#enable-or-disable-the-feature) the
   `:gitaly_go_postreceive_hook` feature flag.
 
 ## Custom error messages