Use cloudfront secure headers

[ConnorKirk]

Issue #, if available:

40

Description of changes:

This PR updates the sample to use CloudFront Response Headers, rather than a Lambda at Edge function to add Security Headers to responses.

• [x] Add CloudFront ResponseHeaderPolicy to template
• [x] Remove Lambda At Edge from template
• [x] Remove other references to lambda at edge from sample (readme, CI etc)
• [x] Update README and architecture diagram

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[HieronymusLex]

the diagram should be using the most up to date architecture icons

[ConnorKirk]

I've updated the diagram to use the latest icon set