cmake: Require Qt 6 to build GUI

.github/ISSUE_TEMPLATE/good_first_issue.yml

@@ -28,7 +28,7 @@ body:
     id: useful-skills
     attributes:
       label: Useful Skills
-      description: For example, “`std::thread`”, “Qt5 GUI and async GUI design” or “basic understanding of Bitcoin mining and the Bitcoin Core RPC interface”.
+      description: For example, “`std::thread`”, “Qt6 GUI and async GUI design” or “basic understanding of Bitcoin mining and the Bitcoin Core RPC interface”.
       value: |
         * Compiling Bitcoin Core from source
         * Running the C++ unit tests and the Python functional tests