Prepare PG19 nightlies with EL8 and OL8 exclusions

Replace the global nightly PostgreSQL 19 exclusion with platform-specific exclusions. Release exclusions and workflow configuration are unchanged. Hold the draft PR until PostgreSQL 19 GA, GA builder images, and Citus PG19 nightly readiness.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 1ed7b58a-0ba9-4f47-a1de-3ee665bae7f2

Author: ihalatci-msft

pg_exclude.yml

@@ -1,6 +1,7 @@
 exclude:
   nightly:
-    all: [19]
+    el/8: [19]
+    ol/8: [19]
   release:
     debian/trixie: [12,13]
     debian/bookworm: [12,13]   # if PGDG dropped 12/13 there