The traffic-light buttons effectively replace the three-dot multitasking button that previously appeared at top-centre of open App windows.
The only available method to disable (hide) the new multitasking control is to completely disable multitasking from iPad settings - although this has the obvious consequence of disabling the ability to have multiple open App windows.
Settings > Multitasking & Gestures - from the available options, select Full Screen Apps

With release of iPadOS 26, many Apps will doubtless be updated over coming months to best integrate with the new version of iPadOS - just as they did when the previous iterations of iPadOS multitasking were introduced.