Remove the Flatpak build

- Delete the manifest and the Flatter deploy workflow

- Drop the Flatpak entries from .gitignore

- The workflow's container image only ever shipped the KDE 6.5 SDK, so
  the build had been failing since the runtime was bumped to 6.7

Author: juzzlin

.github/workflows/Flatter.yml

@@ -34,11 +34,6 @@
 build*
 *.user
 
-# Flatpak
-.flatpak
-.flatpak-builder
-build-dir
-
 # VS Code
 .vscode
 

.gitignore

@@ -69,6 +69,8 @@ Bug fixes:
 
 Other:
 
+* Drop the Flatpak build: the manifest and the Flatter workflow are gone
+
 7.0.0
 =====