Add changenotes for recent template changes.

[freakboy3742]

Adds change notes for:

• Don't reopen stdout against the console if it's already redirected briefcase-windows-VisualStudio-template#126 - altering console redirection for Windows apps
• Set CXX as well as CC for the app_packages build briefcase-linux-flatpak-template#85 - including a fix for CXX in the flatpak build environment.

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