Remove large blurs

[delucis]

This PR aims to close #1274

In several places we had a very large (160px) blur on decorative background elements. These look great, but do not perform well across browsers. In #1228 I had already reduced blur on the hero image to just 16px to improve Safari performance, but didn’t notice these other large blurs at the time.

This PR

• Replaces the two instances of 160px blur with an alternative approach using a radial-gradient mask on the element to simulate a blurred fall-off of intensity. To the trained eye, these are subtly different effects, but they are close enough and I think the trade-off is justified to address poor performance.
• Removes several places we were using backdrop-blur unnecessarily (the backdrop was already something blurry or a flat colour so recalculating additional blurs was superfluous)
• Removes an old component we were no longer using

Browser Test Checklist

I have tested this PR on at least three of the following browsers:

• Chrome / Chromium
• Firefox
• Android Firefox
• Safari
• iOS Safari

[netlify]

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Lighthouse	5 paths audited Performance: 98 (🔴 down 1 from production) Accessibility: 100 (no change from production) Best Practices: 92 (🔴 down 2 from production) SEO: 100 (no change from production) PWA: - View the detailed breakdown and full score reports

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[sarah11918]

I am only on Chrome and can't perceive a difference between the deploy preview and live docs, so certainly nothing appears to have gone wrong in this PR for me!