Wrong default branches

[daniel-starke]

The following files use to the master branch of the referenced repository instead of the main branch:

• projects/arcus.cmake
• projects/cura.cmake
• projects/cura-binary-data.cmake
• projects/curaengine.cmake
• projects/libnest2d.cmake
• projects/pynest2d.cmake
• projects/savitar.cmake
• projects/uranium.cmake

Please correct the branch name in these files accordingly.