Reverts: flutter/flutter#145087
Initiated by: Jasguerrero
Reason for reverting: failing on accessibility checks b/329548765
Original PR Author: bleroux

Reviewed By: {justinmc}

This change reverts the following previous change:
## Description

`InputDecorator` adds a 8.0 gap beetween the container and the helper text.
From the Material 3 specification, this gap should be 4.0.
See https://m3.material.io/components/text-fields/specs#0c5c8d6d-2169-4d42-960c-51f6ee42eb57.

This PR sets the correct values for M3 without changing the value for M2.

| Before | After | M3 Spec |
|--------|--------|--------|
| ![image](https://github.com/flutter/flutter/assets/840911/9947f334-d98f-4f7e-9da7-ca6d5c0770ac) | ![image](https://github.com/flutter/flutter/assets/840911/081dec4b-eb9f-4eee-a7dc-2538e7110ff0)| ![image](https://github.com/flutter/flutter/assets/840911/c8c8f045-3b79-43a5-a1a3-cc6d5460644f) |

If this change is accepted, a future step will be to make this value configurable, probably by `InputDecorationTheme`.

## Related Issue

Fixes https://github.com/flutter/flutter/issues/144984

## Tests

Updates a value used by several existing tests.