2.9 : Linux CPN AppImage SSL packaging

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Fixes #4424, fixes #4316

Summary of changes:

• Use Ubuntu 20.04 (focal), Qt 5.15.2 add extra qt dependencies for linuxdeploy build-edgetx#16 added two packages, as well as a hint for the build system as to where SSL libraries located, and I missed that when bringing chore(cpn): update to Qt 5.15.2 and rework application packaging #4270 over to 2.9 via chore(cpn): 2.9 Win32 Companion bandaid #4309 (which was needed due to upsteam changes). Should only need one of those packages and the hint for 2.9... it appeared to detect locally... need GH to repo that now. Updating edgetx-dev would be the better fix, but this is out of sync with the state of main.

edit: Looking at the output file, looks like more is needed. hm... this is still showing...
> -- Could NOT find OpenSSL, try to set the path to OpenSSL root folder in the system variable OPENSSL_ROOT_DIR (missing: OPENSSL_CRYPTO_LIBRARY OPENSSL_INCLUDE_DIR)

some time later:

• Found OpenSSL: /usr/lib/x86_64-linux-gnu/libcrypto.so (found version "1.1.1") 🥳 🚀

Known limitation: dfu-util is now correctly packaged, and with the correct permissions. Companion currently does not use it, and instead is hard-coded to use the system /usr/bin/dfu-util rather than the packaged dfu-util. However there is little point resolving this at present, as it appears from testing that dfu-util cannot actually access a radio if dfu-util is not installed on the host OS. Thus the message that dfu-util is missing should prompt the user to install it if they want to use DFU with Companion. If this is something we can resolve in the future then the packaged version should naturally be changed.

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Win32, Win64 and Linux targets functional for update/downloading (SSL), DFU updates (Linux may require host level dfu-util), and read/write of models. MacOS build only had superficial changes and dfu-util is still present in the bundle.