fix(header-panel): v10 backport - allow vertical overflow when expanded

[tay1orjones]

Ref #17159
v10 backport of #17160:

When HeaderPanel is expanded, the content is not vertically scrollable if there is overflow. See the original bug report for repro steps.

The root cause is that the --header-panel selector has overflow: hidden for the transition. However, the expanded state should allow vertical scrolling as needed.

The proposed fix is to set overflow-y: auto on the expanded state to only afford a scrollbar as needed.

Note that this fix should ideally be backported to v10.

Before

### After #### Changelog **Changed**

• fix(header-panel): allow vertical overflow when expanded

Testing / Reviewing

• Test the PR Preview: https://deploy-preview-17160--v11-carbon-react.netlify.app/?path=/story/components-ui-shell-header--header-w-actions-and-switcher