Add changes for v2.7.6

I picked from https://github.com/ruby/json/blob/v2.7.x/CHANGES.md

Author: y-yagi

CHANGES.md

@@ -8,6 +8,10 @@
 * Bump required_ruby_version to 2.7.
 * More performance improvments to `JSON.dump` and `JSON.generate`.
 
+### 2024-11-04 (2.7.6)
+
+* Fix a regression in JSON.generate when dealing with Hash keys that are string subclasses, call `to_json` on them.
+
 ### 2024-10-25 (2.7.5)
 
 * Fix a memory leak when `#to_json` methods raise an exception.