Rename namespace from Devices to Device

[networkfusion]

Description

Renames the namespace from System.Devices to System.Device
(will require the repo to be renamed.

Motivation and Context

Follows latest preview .net 5 namespace as found at https://github.com/dotnet/iot/tree/master/src/System.Device.Gpio/System/Device

How Has This Been Tested?

No tests run yet.

Screenshots

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Config and build (change in the configuration and build system, has no impact on code or features)
• Dependencies (update dependencies and changes associated, has no impact on code or features)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

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