chore: lead release notes with curated highlights

Hide the low-signal Code Refactoring section from the auto-changelog and
require a curated Highlights section, so releases lead with what matters.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 936105978

.github/release-please-config-v1.json

@@ -25,7 +25,8 @@
         },
         {
           "type": "refactor",
-          "section": "Code Refactoring"
+          "section": "Code Refactoring",
+          "hidden": true
         },
         {
           "type": "docs",

.github/release-please-config.json

@@ -22,7 +22,8 @@
         },
         {
           "type": "refactor",
-          "section": "Code Refactoring"
+          "section": "Code Refactoring",
+          "hidden": true
         },
         {
           "type": "docs",