Replace aria label with text for nav button

[tochwill]

Resolves #2950

Overall change:
The BBC a11y check on CI is failing with the following error:

• Forms: Labelling form controls: Fields must have labels or titles
  • Button has no text: //div[@id='root']/header/nav/div/div[1]/button

This is because the button contains no text, and relies instead on an aria-label. We can use the VisuallyHiddenText component instead, which will pass the a11y acceptance criteria.

Code changes:

• A bullet point list of key code changes that have been made.
• When describing code changes, try to communicate how and why you implemented something a specific way, not just what has changed.

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• I have assigned myself to this PR and the corresponding issues
• Automated jest tests added (for new features) or updated (for existing features)
• This PR requires manual testing

[AlistairGempf]

Looks good.