ci: Use KF6 ECM

This is needed to build frameworks that (indirectly) depend on plasma-wayland-protocols to build on the CI

Otherwise plasma-wayland-protocols pulls in ECM from 'kf5' branch into the build

Since KF6 ECM is still compatible with Qt5 this doesn't affect the ability to build against Qt5
diff --git a/.kde-ci.yml b/.kde-ci.yml
index 3114d0c..d21fb17 100644
--- a/.kde-ci.yml
+++ b/.kde-ci.yml
@@ -1,7 +1,7 @@
 Dependencies:
 - 'on': ['@all']
   'require':
-    'frameworks/extra-cmake-modules': '@stable'
+    'frameworks/extra-cmake-modules': '@latest-kf6'
 
 Options:
   test-before-installing: True