Use visually hidden text instead of aria-label for navigation menu button.

[tochwill]

Describe the bug
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.

Ensure that the correct screen reader behaviour is still present after this change is made.

To Reproduce
Steps to reproduce the behaviour:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behaviour
A clear and concise description of what you expected to happen.

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Testing notes

• This bug fix is expected to need manual testing.

Additional context
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