Update GitHub files for C#/Unity SDK (#2952)

Co-authored-by: Zeke Foppa <[email protected]>

Author: bfops

.github/pull_request_template.md

@@ -1,3 +1,7 @@
+# Migration note
+
+We are in the process of moving from the `com.clockworklabs.spacetimedbsdk` repo to the `sdks/csharp` subdirectory of [SpacetimeDB](https://github.com/clockworklabs/SpacetimeDB). **Any new changes should be made there**. The `com.clockworklabs.spacetimedbsdk` repo will only be updated on release. Apologies in advance for any sharp edges while the migration is in progress.
+
 # Notes for maintainers
 
 ## `SpacetimeDB.ClientApi`

.github/workflows/check-pr-base.yml

@@ -10,5 +10,4 @@ The Unity SDK uses the same code as the C# SDK. You can find the documentation f
 
 There is also a comprehensive Unity tutorial/demo available:
 - [Unity Tutorial](https://spacetimedb.com/docs/unity/part-1) Doc
-- [Unity Demo](https://github.com/clockworklabs/Blackholio) Repo
-- 
+- [Unity Demo](https://github.com/clockworklabs/SpacetimeDB/tree/master/demo/Blackholio) Repo