fix: Update mocks to use specific date strings

[camdecoster]

Description

Update four mocks to make date strings unambiguous.

Closes #7969.

Changes

• Update mock data

Testing

• Be on master
• Open plotly devtools
• Render the following mocks with this command: npm run baseline axes_breaks domain_ref_axis_types ohlc_first range_selector_style
  • test/image/mocks/axes_breaks.json
  • test/image/mocks/domain_ref_axis_types.json
  • test/image/mocks/ohlc_first.json
  • test/image/mocks/range_selector_style.json
• Look at the git status and note that they all change
• Switch to this branch
• Run the render command again
• Note that the images now show no diff

Notes

• I've spent some time chasing this issue as a bug that I introduced when it was unrelated
• Fixing this so I don't do that again
• Another way to see the issue is to run the render command on master, but setting the timezone to UTC: TZ=UTC npm run baseline.... This runs the render in the same TZ as CI, and no diff is present.

[emilykl]

🎉