Update dot style in DORA lead time graphs

What does this MR do?

Updates the DORA 4 lead time graphs (both project and group-level) to always render each data point with a blue dot. This makes it easier to see isolated data points on the graphs.

Screenshots

Before	After

Hover style

Updates

Since capturing the screenshots/GIF above, the dot size has been reduced from 12px to 8px, so the dots will render a bit smaller than what is shown above. Now they look like this:

Related to #329645 (closed)