docs: update nodejs required verson, 'app' caps in uia2 and xcuitest

[KazuCocoa]

Proposed changes

• addresses feat: Make it possible to start UIA2 server without an app or a package id appium-uiautomator2-driver#337 in app capability
• update required NodeJS version

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• Bugfix (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 not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

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• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

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[coveralls]

Coverage remained the same at 89.391% when pulling b1fc260 on KazuCocoa:km/update-nodever-uia2 into 71b9876 on appium:master.