Use KDE SDK, Runtime and KF6

[sungsphinx]

[!IMPORTANT]
This needs testing, please do not mark as ready yet.

Uses changes from:

https://github.com/flathub/org.libreoffice.LibreOffice/pull/253 (Switch to KDE Runtime/SDK and Building with KF5/QT5 VCL's Backends by @EliasOfWaffle, based on @CarlSchwan's PR)

[flathubbot]

Started test build 107627

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Build 107627 failed

[sungsphinx]

bot, build

[flathubbot]

Queued test build for org.libreoffice.LibreOffice.

[flathubbot]

Started test build 107628

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Build 107628 failed

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Started test build 107629

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Started test build 107653

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Build 107653 failed

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Started test build 113168

[sungsphinx]

Still fails, and it is still related to standard headers (stdlib.h/cstdlib). Not sure what to do :/

[flathubbot]

Build 113168 failed

[EliasTheGrandMasterOfMistakes]

Is not better to use qt6 and KF as module for the default runtime instead of change runtime?

[EliasTheGrandMasterOfMistakes]

This is the same build error from older kde runtime

[flathubbot]

Started test build 113767

[sungsphinx]

Is not better to use qt6 and KF as module for the default runtime instead of change runtime?

It could technically be possible, but I mean, just look at the size of the runtime/SDK json... https://invent.kde.org/packaging/flatpak-kde-runtime/-/blob/qt6.7/org.kde.Sdk.json.in, and I don't think it would change much anyway.

[sungsphinx]

bot, build

(hopefully it will stop the previous build)

[flathubbot]

Queued test build for org.libreoffice.LibreOffice.

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Build 113767 failed

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Started test build 113771

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Started test build 113776

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Started test build 113792

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