feat!: Disallow blue Spotlight and make purple the default

[VincentSmedinga]

Describe the pull request

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What

This removes the blue option for the colour of a Spotlight.

Why

We don’t want the appearance of Spotlight to interfere with that of Footer.

How

• Changed the default to purple.
• Applied the default to the main component class.

Checklist

Before submitting your pull request, please ensure you have done the following. Check each checkmark if you have done so or if it wasn't necessary:

• Add or update unit tests
• Add or update documentation
• Add or update stories
• Add or update exports in index.* files
• Start the PR title with a Conventional Commit prefix, as explained here.

Additional notes

• The tests will succeed after completing feat: Let Footer set its own background colour #1797 and merging develop into this branch.