Update and normalize changelog

We've normalized the format across KD* repos so automated release
tooling can parse it use it for release notes

Author: iamsergio

Changelog

@@ -1,13 +1,13 @@
-* v1.2.0 (unreleased)
+* v1.2.0
  - KDSingleApplication now looks for Qt6 by default, rather than Qt5. If your Qt5
    build broke, pass -DKDSingleApplication_QT6=OFF to CMake.
+ - Do not exceed maximum socket name length
+ - Fix maximum socket length on Windows
+ - CI: Add FreeBSD and OpenBSD
 
-* v1.1.1 (unreleased)
- -
-
-* v1.1 (22 December 2023)
+* v1.1
  - Add option making session ID part of the socket name optional
  - Fix dynamic library versioning
 
-* v1.0 (21 July 2023)
+* v1.0
   - Initial Release