Prepare sdk/internal v1.9.0 for release (#22991)

Author: chlowell

sdk/internal/CHANGELOG.md

@@ -1,17 +1,11 @@
 # Release History
 
-## 1.9.0 (Unreleased)
+## 1.9.0 (2024-06-06)
 
 ### Features Added
 * Added `test/credential` package containing a credential factory for live and recorded tests
   and a fake credential for offline tests.
 
-### Breaking Changes
-
-### Bugs Fixed
-
-### Other Changes
-
 ## 1.8.0 (2024-05-09)
 
 ### Breaking Changes