drop Electron v5 and v6 support for the next major release

[shiftkey]

This aligns us with the current supported versions of Electron: https://www.electronjs.org/docs/tutorial/support

This simply requires dropping the two versions from these lines in package.json:

node-keytar/package.json

Lines 39 to 40 in 60aa824

	"prebuild-electron": "prebuild -t 5.0.0 -t 6.0.0 -t 7.0.0 -t 8.0.0 -t 9.0.0 -r electron --strip",
	"prebuild-electron-ia32": "prebuild -t 5.0.0 -t 6.0.0 -t 7.0.0 -t 8.0.0 -t 9.0.0 -r electron -a ia32 --strip",

[aurelii]

On it.

[shiftkey]

This happened in #311