BUG - Exported KML File with Encoding UTF-16, but file is encoded as UTF-8

#### Issue details
When exporting a KML File from Mission Planner, the xml options explicitly configure xml as being encoded utf-16:

However the file itself is encoded as utf-8 instead.
Please address this as it affects many users. Thanks

#### Version
1.3.76

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

#### Airframe type
N/A

#### Hardware type
N/A

#### Logs
N/A

No file or test is named. Start by locating Mission Planner's KML export entry point and inspect how the XML declaration and output bytes are produced; done means the exported file's declared encoding matches its actual encoding, with the existing export behavior verified on all platforms.

Written by the indexing model from the issue text.