Remove tabindex when non-interactive map

[kumilange]

Handling the good first issue #12822, removing tabindex attribute when map is set as non-interactive.

[Before]

[After]

Note about Attribution

Since Attribution which has links and is tabbable seems to be a must to have on map, its behavior is still kept as it is like this: http://g.recordit.co/ET7dF5c9Ny.gif

Launch Checklist

• briefly describe the changes in this PR
• include before/after visuals or gifs if this PR includes visual changes
• write tests for all new functionality
• manually test the debug page
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Remove tabindex attribute when map is non-interactive</changelog>

---

closes #12822

[CLAassistant]

All committers have signed the CLA.

[stepankuzmin]

Hi, @kumiko-haraguchi

Thanks for the contribution, it looks good to me 👍