Fix scroll gradient in modals

[elycheea]

What package(s) are you using?

• Carbon for IBM Products (or Carbon for IBM Cloud & Cognitive)
• Legacy/CDAI
• Legacy/Security

Detailed description

Describe in detail the issue you're having.

Scroll gradient is not positioned correctly in the TagSet overflow modal. Scroll gradient is not present in AboutModal but believe that it is intended to be.

TagSet overflow	AboutModal

Is this issue related to a specific component?

TagSetModal, AboutModal.
These likely should just be using hasScrollingContent from the Carbon composed modal.

What did you expect to happen? What happened instead? What would you like to
see changed?

Scroll gradient should be positioned to the bottom of the TagSet modal (likely a result of not having buttons in the TagSetModal.) AboutModal should also use scrolling content when content is long.

What version of the @carbon/ibm-products (or @carbon/ibm-cloud-cognitive)
package are you using?
v2.26.0

Steps to reproduce the issue

1. Navigate to TagSet story

2. Click or activate the overflow (+xxx) to open the modal

3. Modal should open with the scrolling overflow floating

4. Navigate to AboutModal story

5. Set content to long content

6. Modal does not display a scrolling overflow

Additional information

• Screenshots or code
• Notes