Fix months count in Enhanced Issue Analytics chart x-axis title

What does this MR do and why?

Fixes error in months count in the Enhanced Issue Analytics chart x-axis title – "Last x months" is off by a month.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Make sure you have a GitLab Ultimate license
2. Feature.enable(:issues_completed_analytics_feature_flag)
3. Go to a group/project with issues: http://{LOCAL_INSTANCE}/groups/{GROUP_PATH}/-/issues_analytics or http://{LOCAL_INSTANCE}/{GROUP_PATH}/{PROJECT_PATH}/-/analytics/issues_analytics
4. Verify that the chart's x-axis title is correct.

MR acceptance checklist

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