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

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Library Version

_No response_

### Expected Behavior

Not to use Schedule type for this field

### Current Behavior

uses overly broad Schedule type

### Reproduction Steps

n/a

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### AWS CDK Library version (aws-cdk-lib)

n/a

### AWS CDK CLI version

n/a

### Node.js Version

n/a

### OS

n/a

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Start at packages/@aws-cdk/aws-elasticache-alpha/lib/serverless-cache.ts around the dailyBackupTime field and inspect how the current events.Schedule type is exposed. Decide between a validated time-of-day string and a narrower helper type, then verify that the field accepts only the intended HH:MM value and no longer exposes unrelated schedule configuration.

Written by the indexing model from the issue text.