We fixed a limit at 150m for the flight altitude for security reasons. For example this prevents the Capture app from disconnecting with the drone, and thereby loosing control over it.
There remains a possibility to fly at higher altitude with the “Free flight mission” mode. Please note that this mode is designed for advanced users with experience flying drones manually.
It is true that for forested areas and dense vegetation, it is recommended to have a GSD higher than 10cm/pixel. As you have probably read here: https://support.pix4d.com/hc/en-us/articles/202557459
10cm/pixel is achieved with an altitude close to 250m for the Phantom 3 Pro which is indeed not possible with Pix4Dcapture.
In your case if you still want to do a Grid Mission, we would recommend to increase the overlap as recommended in our article above and to fly at 150m. Otherwise you could flight the drone manually at 250m using the Free Flight Mission and pictures will be taken at given horizontal/vertical intervals.