Fix Calendar component PREV/NEXT month, year, and "Go to today" handlers firing twice

[KevinTCoughlin]

Pull request checklist

• Addresses an existing issue: Fixes DatePicker: pressing space/Enter on Previous/Next month/year arrow buttons goes 2 months at a time #4620
• Include a change request file using $ npm run change

Description of changes

In #3741 we refactored all the Calendar <span/> nodes to <button/>, which was a good change 👍 . However, React invokes onClick for KeyboardEvent if the keyCode is SPACE. Since each <button/> node had an onClick and onKeyDown it would invoke the PREV/NEXT month, year, and also "Go to today" callbacks twice.

Solution Options:

I went with # 1, but there's an alternative solution:

1. If we wish to still allow for ENTER to invoke the <button/> then the change is what is in this PR. We need to only invoke the onKeyDown callbacks if the key was ENTER.
2. If we only want SPACE to invoke the callback then we can safely remove the onKeyDown handlers.

Since # 2 would would change prior behavior so I left went with option # 1.

Focus areas to test

• Use the Calendar component's PREV/NEXT month and year buttons with both the mouse and keyboard.
• The time unit either month or year should change 1 unit per invocation.
• "Go to today" should also only fire once, but you'll likely need to set a break-point to verify.

Note: that the UI doesn't update when "Go to today" is invoked. Looks like a preexisting bug on master that I'll look into but may file separately. I assume the UI updated at one point, but need to check behavior in prior commits.