remove "Beta"

[jasongrout]

See #4898 for the discussion.

[blink1073]

This is the only other relevant use of beta that I found: ./packages/help-extension/src/index.tsx:273: let release = 'Beta Release Series';

[jasongrout]

Thanks.

I also fixed the about dialog - the styling from #2550 didn't work anymore because our DOM structure changed, so we had an overflow: auto scrollbar problem. Here's the change in this PR:

Before:

After:

[blink1073]

Looks great, thanks!