Fix date spec failure merge_request_analytics/graphql/throughput_chart_query_builder_spec.js

What does this MR do?

In the spec we want the beginning of the month.

Due to we use current date as prototype, and manipulate month first, during the 31st, when we want a month with only 30 days, the month will increase unintentionally.

This can either be fixed if we manipulate days first, or just use Date.new(year, month, day).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Closes #243437 (closed)