feat(escalating-issues): Add escalating groupinbox reason (#45964)

## Objective:

Create new enum `escalating` for GroupInboxReason

src/sentry/models/groupinbox.py

@@ -31,6 +31,7 @@ class GroupInboxReason(Enum):
     REGRESSION = 2
     MANUAL = 3
     REPROCESSED = 4
+    ESCALATING = 5
 
 
 class GroupInboxRemoveAction(Enum):