refactor: use DATE_TRUNC for Elasticsearch time grain

[mikelv92]

SUMMARY

As discussed in #25683 this is a follow-up PR to refactor the Elasticsearch time grain expressions to use DATE_TRUNC instead of HISTOGRAM in order to have consistency.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TESTING INSTRUCTIONS

To verify that the time grains still work:

• Create a database connection to an Elasticsearch cluster
• Create an index in the Elasticsearch cluster. The index documents should have a date field and at least another field to be used in a metric
• Create a dataset using the Elasticsearch database, and the choose the index created previously as the table
• Create a time series chart and use the date field in the x axis
• Select various time grains from the drop down menu and verify that the chart shows the time series data

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[john-bodley]

Thanks @mikelv92 for the refactor. LGTM.