Bug - [Modal] - [update the position prop type]

[tlabaj]

Describe the problem
The position prop type declaration is not consistent with other similar props. We should add default as a possible value to the prop.

It should be:
position?: 'default' | 'top';

The default value of the prop should be set to default

(see the following PR comment for more details: #8117 (comment))

What is your product and what release date are you targeting?

Any other information?