Update our browser support approach and documentation to align with GOV.UK

[anandamaryon1]

Our browser support policy has not been updated in some years: https://github.com/nhsuk/nhsuk-frontend/blob/main/docs/contributing/testing.md

Gov.uk have recently dropped support for IE11 javascript: https://github.com/alphagov/govuk-frontend/blob/main/docs/contributing/browser-support.md

We should consider updating our approach to follow their lead.

[anandamaryon1]

Arrange a meeting to discuss

[sarawilcox]

See Remove legacy browser support with <script type="module"> #1260. In particular the guidance bit: nhsuk/nhsuk-frontend@4bd0f60.

[sarawilcox]

Do we want for next service manual release?