Update the runtime version from 48 to 50, and more

[yakushabb]

• Update the runtime version to 50
• Add the required python3-six module
• Use the blueprint-compiler module from the runtime
• Drop ineffective cleanup commands
• Improve the cleanup commands to reduce the Flatpak size

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✅ Test build succeeded. To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/272233/com.github.tenderowl.frog.flatpakref

Built for aarch64 and x86_64 architectures.

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[yakushabb]

@amka, the runtime version 48 reached end-of-life. Would you review this PR? Thanks.