Update SECURITY.md to conform to semver

[is2ei]

The version 1.4.x is not supported, right?
It would be confusing a bit, so I'd like to make it clear.

const semver = require('semver')

console.log(semver.satisfies('2.1.2', '2.x.x')); // true
console.log(semver.satisfies('1.5.8', '1.5.x')); // true
console.log(semver.satisfies('1.4.3', '<1.4')); // false

console.log(semver.satisfies('1.4.3', '<=1.4')); // true

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View rendered SECURITY.md

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