Update for Pinta 2.1

[cameronwhite]

• Upgrade to .NET 7
• Update to the latest nuget package versions
• Remove some icon generation and copying. This broke with upstream commit 921292f2f1b44e9f53f159460695c164487d7e34, but shouldn't be necessary anymore since Pinta installs the application icons to the correct location now.

[flathubbot]

Started test build 14905

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Build 14905 failed

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Started test build 14906

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Started test build 14907

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Started test build 14908

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Started test build 14909

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Started test build 14920

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Started test build 14926

[flathubbot]

Build 14926 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/127348/com.github.PintaProject.Pinta.flatpakref

[flathubbot]

Started test build 15102

[flathubbot]

Build 15102 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/127524/com.github.PintaProject.Pinta.flatpakref