Productivity Analytics Scatterplot: Rename y-axis label to "Days to merge"

Currently, the y-axis label on for the scatterplot on the Productivity Analytics page says Days. As mentioned by @ameliabauerly in !20462 (comment 250265062) this can be confusing and is inconsistent with the metric label in the dropdown above the chart.

We should consider renaming the y-axis label to Days to merge.

Also, we're displaying Hours for all time related metrics (except for Days to merge), see the table below. Maybe that's something we should revise as well?

Dropdown label	Y-axis label
Days to merge	Days
Time from first commit until first comment	Hours
Time from first comment to last commit	Hours
Time from last commit to merge	Hours
Number of commits per MR	Number of commits per MR
Number of LOCs per commit	Number of LOCs per commit
Number of files touched	Number of files touched