ok, so I tried to install 2.6.0 and boom, there is the same problem as you guys have
I looked into sources, did a little digging and I have a solution for all of you
disadvantages:
1. most probably it's valid for Android users only (I don't even know if the same problem is on iOS version)
2. you won't be able to use internet connection while using DJI GO
so:
1. download attached zip, unzip it and copy those 2 files somewhere to your phone (so you can use them again and again)
2. run DJI GO and cache your maps that you need
3. close DJI GO completely
4. turn off internet connection
5. use some file browser (I recommend Total Commander) and navigate to internal SD card's folder \Android\data\dji.pilot\cache (it may seem familiar as we put .DJI.configs into \Android\data\dji.pilot\files)
6. you can see there are the same 2 files - replace them with the files you copied into your phone
7. enjoy DJI GO with no forced upgrade
I think I should be able to patch apk in a way that this message will never be displayed, even when phone is connected to the internet
if anybody would like to try, choose version number to patch