This PR improves the distance between the label and the icon in the Tab widget. 
I updated the margin to 2 pixels, taken from the Figma design page for Material 3. On Material 2 I left the default value of 10 pixels.

Related to #128696 (In particular, the distance between label and icon)

Here are some screenshots for comparison. I looked a bit into the other mentioned issue of the tab height not following the M3 spec. Flutter uses 72 and the spec uses 64. But because Tab is a PreferredSizeWidget, I don't think there is an easy way to provide a different size depending on `ThemeData.useMaterial3`, because there is no `BuildContext` available.
I provide a sample image for the 64 height as well for context on the linked issue, even though it's not part of the PR changes.

The screenshots are taken side by side with the image at: https://m3.material.io/components/tabs/guidelines

## Original

![original](https://github.com/flutter/flutter/assets/22084723/f52d46bb-eaf9-4519-976e-9ea07c021e14)

## New (tab height = 72, Flutter default for 8 years)

![new_72](https://github.com/flutter/flutter/assets/22084723/8c9d3510-eaca-4b7d-92d8-0d06a7e75136)

## New (tab height = 64, M3 spec)

![new_64](https://github.com/flutter/flutter/assets/22084723/f8811b70-766f-4a4f-b069-33673b1e3744)