aws-rds: Exclude caret (`^`) from allowed password pattern

### Describe the bug

The exclusion list is maintained here:

https://github.com/aws/aws-cdk/blob/fee06382e0f36f7f241196b67a23a760eb9da099/packages/aws-cdk-lib/aws-rds/lib/private/util.ts#L11-L19

The caret (`^`) character is not allowed by Go's `net/url` parser and it will fail with:

> net/url: invalid userinfo

### Regression Issue

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

### Last Known Working CDK Library Version

N/A

### Expected Behavior

The caret (`^`) character to be excluded.

### Current Behavior

It is not excluded.

### Reproduction Steps

N/A

### Possible Solution

Exclude the character.

### Additional Information/Context

I think this might break existing secrets as they will get regenerated, so it might need to be a feature flag for the future.

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

2+

### AWS CDK CLI version

N/A

### Node.js Version

N/A

### OS

N/A

### Language

TypeScript

### Language Version

N/A

### Other information

_No response_

Start with packages/aws-cdk-lib/aws-rds/lib/private/util.ts at the referenced exclusion list and review how the password pattern is used. Confirm that caret is excluded without breaking existing secret behavior, and determine whether the issue's feature-flag concern requires maintainer guidance before considering it done.

Written by the indexing model from the issue text.