fix: remove 1px line from gatsby-announcer on 100vh pages

[madalynrose]

Description

The 1px height style on the gatsby-announcer div adds a 1px line to the bottom of 100vh pages. We want the announcer element to be visually hidden on the page, and have followed best practices from WebAIM.

width:1px; height:1px; overflow:hidden; tells the browser to make the element 1px by 1px in size and to visually hide everything that does not fit into these dimensions. While this is a likely a little overkill and can probably be omitted in most circumstances, there are a few instances where positioning may be disabled, but all other styles remain enabled. In this case, the element will remain in its original position, but will only take 1 pixel of space.

To remove the line at the bottom of 100vh pages, we've added top: 0 to the list of styles for the announcer element.

Related Issues

Fixes #20728

[pieh]

Looks good! Thanks!