Keep boundary PostgreSQL versions in CI matrix

Revert the matrix to the boundary versions [14, 18] for lower
maintenance, rather than enumerating every supported major. The
non-blocking PostgreSQL 19 readiness job is retained.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UYdvaxjc8p2XSCc3hTuHCV

Author: claude

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
       matrix:
         ruby: ["3.3", "4.0"]
         rails: ["7.2", "8.1"]
-        postgres: ["14", "15", "16", "17", "18"]
+        postgres: ["14", "18"]
         include:
           - ruby: "4.0"
             rails: "main"

CHANGELOG.md

@@ -10,7 +10,6 @@ changelog, see the [commits] for each version via the version links.
 [Unreleased]: https://github.com/teoljungberg/fx/compare/v0.11.0...master
 
 - Internal refactorings / improvements
-  - Test the full range of supported PostgreSQL versions (14-18) in CI
   - Add a non-blocking CI job for the upcoming PostgreSQL 19 release
 
 ## [0.11.0]