06_leapYears/solution: Fix solution.js to align with README.md hint

… of this exercise.

Because

This PR changes the solution for Exercise 06 - leapYears, to align the solution with what has been taught up until this point of the course.

Changes solution.js to use if statements in combination with the && operator in order to match the hint in README.md and to better align with the concepts taught up to this point in the course.

Closes #482

[x] I have thoroughly read and understand The Odin Project Contributing Guide
[x] The title of this PR follows the location of change: brief description of change format, e.g. 01_helloWorld: Update test cases
[x] The Because section summarizes the reason for this PR
[x] The This PR section has a bullet point list describing the changes in this PR
[x] If this PR addresses an open issue, it is linked in the Issue section
[x] If this PR includes any changes that affect the solution of an exercise, I've also updated the solution in the /solutions folder