docs: add CHANGES entry for pointer-sign warning fixes (#426, #427)

[thegushi]

Summary

• Adds a CHANGES-202605.md entry under "Build system and portability" documenting fix: resolve char/u_char pointer-sign compiler warnings #426 and fix: resolve char/u_char pointer-sign warnings in libopendmarc test suite #427 (char/u_char pointer-sign warning cleanup in libopendmarc, the milter, and the test suite).