clean up outstanding V8 warnings about deprecated APIs

[shiftkey]

• no V8 build warnings on Windows
• no V8 build warnings on macOS
• no V8 build warnings on Linux

[shiftkey]

The last remaining V8 warning on Windows is not relevant to keytar's usage, and will hopefully go away with a future tooling update:

c:\users\appveyor\.node-gyp\5.0.0\include\node\v8.h(4458): warning C4996: 'v8::WasmModuleObject::SerializedModule': was declared deprecated (compiling source file ..\src\async.cc) [C:\projects\node-keytar\build\keytar.vcxproj]
  c:\users\appveyor\.node-gyp\5.0.0\include\node\v8.h(4380): note: see declaration of 'v8::WasmModuleObject::SerializedModule' (compiling source file ..\src\async.cc)
c:\users\appveyor\.node-gyp\5.0.0\include\node\v8.h(10172): warning C4996: 'v8::Value::ToBoolean': was declared deprecated (compiling source file ..\src\async.cc) [C:\projects\node-keytar\build\keytar.vcxproj]
  c:\users\appveyor\.node-gyp\5.0.0\include\node\v8.h(2387): note: see declaration of 'v8::Value::ToBoolean' (compiling source file ..\src\async.cc)