cloudwatch: `M` shadows both minutes and months, impossible to define relative minutes in `start` param for a GraphWidget or Dashboard

### Describe the bug

Impossible to specify relative minutes in cloudwatch GraphWidget and Dashboard.

I first noticed this by realizing the docs have an issue...

per: https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/aws-cloudwatch/lib/dashboard.ts#L48

```
You can specify start without specifying end to specify a relative time range that ends with the current time. In this case, the value of start must begin with -P, and you can use M, H, D, W and M as abbreviations for minutes, hours, days, weeks and months.
```

How can `M` be both minutes and months?

After a little investigating in the UI and several CDK deployments I believe it is impossible to specify a relative minutes, given that use `M` for the start string always results in months when I deploy a dashboard through CDK.

In addition, when manually modifying the `start` param from the relative calendar dropdown, selecting minutes results in a string like `PT3M` but selecting months results in a string like `PT2160H`.

### Regression Issue

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

### Last Known Working CDK Version

_No response_

### Expected Behavior

The first issue is that `M` should not be valid for both minutes and months. The second issue is that it is impossible to have a relative duration in minutes through CDK currently.

### Current Behavior

Deploy a dashboard with a `start` param of anything with `M`, it defaults to months and there is no way to specify minutes. No errors it just silently gives you months

### Reproduction Steps

Deploy a dashboard with a `start` param of anything with `M`, it defaults to months and there is no way to specify minutes.

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

2.1003.0 (build b242c23)

### Framework Version

_No response_

### Node.js Version

N/A

### OS

N/A

### Language

Python, TypeScript, .NET, Java, Go

### Language Version

_No response_

### Other information

_No response_

Start with the documentation around packages/aws-cdk-lib/aws-cloudwatch/lib/dashboard.ts:48, then trace how the start parameter is handled for GraphWidget and Dashboard. Reproduce the deployed behavior using a relative start containing M and compare it with the PT3M and PT2160H forms reported from the CloudWatch UI. Done means relative minutes can be specified without being silently interpreted as months.

Written by the indexing model from the issue text.