Old Useragent setting causing map viewing to fail

#### Issue details
Due to a hard coded user agent setting located here -> https://github.com/ArduPilot/MissionPlanner/blob/142d36759862805b561e6ce5e471665306613dd0/ExtLibs/GMap.NET.Core/GMap.NET.MapProviders/GMapProvider.cs#L328

We are prevented from viewing any maps inside of mission planner.

#### Version
Latest Stable and Dev/Beta

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
_What type of airframe (flying wing, glider, hex, Y6, octa etc)_

#### Hardware type
_What autopilot hardware was used? (pixhawk, pixracer, PX4FMU etc)_

#### Logs
User agent in question shows up as Microsoft Edge Older Versions(Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36 Edge/17.17074) which ends up being blocked due to corporate policy. Updating the string i linked above to a newer user agent should resolve the issue for those running this in environments with strict network controls in place

Open ExtLibs/GMap.NET.Core/GMap.NET.MapProviders/GMapProvider.cs at the hard-coded user-agent setting around line 328. Update the outdated user-agent string to a newer value and verify that map viewing works in Mission Planner under the affected corporate network policy.

Written by the indexing model from the issue text.