feat(datetime): Migrate standard datetime parsing to whenever library

[devin-ai-integration]

This PR adds unit tests to verify that the datetime helper module correctly uses the 'whenever' parser for ISO/RFC compliant datetime values and falls back to dateutil.parser.parse for other formats.

Link to Devin run: https://app.devin.ai/sessions/2e37cc1080054be58292cfb5cf0551ff

Requested by: Aaron

[devin-ai-integration]

🤖 Devin AI Engineer

Original prompt from Aaron:

In the CDK, create some unit tests that confirm a specific code path when parsing dates using our datetime helper module. Specifically, when parsing a datetime value that exactly conforms to the ISO/RFC specs, I expect the parsing will be done entirely with the `whenever` parser. And for other formats, I expect the parsing will fall back to datetime utils or another method.

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[aaronsteers]

The parameterized test seems to cover these cases. Can we delete this test?

[devin-ai-integration]

Closing due to inactivity for more than 7 days.