elasticache-alpha: dailyBackupTime field uses misleading Schedule type

Describe the bug

The alpha elasticache module uses events.Schedule for dailyBackupTime. The expected elasticache type in the Cfn is just a time of day string HH:MM, but schedule is essentially a full cron interface leading to user potentially configuring a bunch of things that are irrelevant to the field.

https://github.com/aws/aws-cdk/blob/67f09c04ad860ff25494e1e76071c304adcf39b4/packages/%40aws-cdk/aws-elasticache-alpha/lib/serverless-cache.ts#L84

Proposal: Create a new helper type to specify time of day or revert back to expect a string on this field with validation in the construct that it matches expected string format.

Regression Issue

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Last Known Working CDK Library Version

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Expected Behavior

Not to use Schedule type for this field

Current Behavior

uses overly broad Schedule type

Reproduction Steps

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Possible Solution

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Additional Information/Context

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AWS CDK Library version (aws-cdk-lib)

n/a

AWS CDK CLI version

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Node.js Version

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OS

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Language

TypeScript

Language Version

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Other information

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