Downgrade Application refactor changelog to patch

The Application extraction (#2926) is a backwards-compatible internal
refactor: no public method, hook signature, or behavior changed for users or
integrations (the moved queue_reject/generate_key_pair helpers still exist).
Major significance is reserved for breaking changes, so this is a patch.

Claude-Session: https://claude.ai/code/session_01Eam1mCnfuFmXKSAtYFxaua

Author: pfefferle

.github/changelog/refactor-extract-application

@@ -1,4 +1,4 @@
-Significance: major
+Significance: patch
 Type: changed
 
 Improve the internal handling of the Application actor used for server-to-server requests.