fix(dataset): Ensure DATETIME FORMAT is ISO 8601 compliant

[john-bodley]

SUMMARY

The dataset DATETIME FORMAT should be ISO 8601 compliant for lexicographical ordering purposes. This is mentioned throughout the code however the placeholder text for the dataset editor was %Y/%m/%d which is non-compliant and likely lead people astray which resulted in #24113.

#25510 reverts the validation logic, though this PR also ensures that the underlying placeholder text is changed to %Y-%m-%d per the ISO 8601 standard.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

Updated unit tests.

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

cc: @jfrag1

[michael-s-molina]

Blocking this PR until we resolve if it constitutes a breaking change or not.

[john-bodley]

@michael-s-molina I gather because we're now in a breaking window we could merge this PR irrespective of whether it constitutes a breaking change or not.

[michael-s-molina]

@michael-s-molina I gather because we're now in a breaking window we could merge this PR irrespective of whether it constitutes a breaking change or not.

@john-bodley Only breaking changes that were approved by lazy consensus can be merged as someone might have objections during the lazy consensus period. I think the correct process here is to add a card to Punted to 5.0 column so we don't miss this again in the next breaking window.

[john-bodley]

Closing in favor of #25510 which incorporated these changes.