Update internal browser support documentation

[vanitabarrett]

What

Update any documentation in the GOV.UK Frontend repo that references our browser support, e.g: the README.

We should include:

• what our new level of support is
• that we recommend using v4.x.x if you need to support IE
• that our level of support for v4.x.x updates is reduced and will be revisited in future
• what we mean by 'functional' and 'compliant' (if we use that terminology)

Why

We are changing our browser support, so should make sure our documentation is updated. See our browser proposal for more details.

Who needs to work on this

Tech writer

Who needs to review this

Developers

Done when

• GOV.UK Frontend README is updated with our new browser support
• We have documentation on what to do if your service needs to support Internet Explorer
• We have documentation on what updates v4 will receive and for how long
• We have updated any other references to browser support or Internet Explorer as necessary

[EoinShaughnessy]

Other docs in repo that mention IE:

• https://github.com/alphagov/govuk-frontend/blob/main/docs/contributing/testing.md
• https://github.com/alphagov/govuk-frontend/blob/main/docs/contributing/resize-text-in-browsers.md
• https://github.com/alphagov/govuk-frontend/blob/main/docs/contributing/coding-standards/css.md
• https://github.com/alphagov/govuk-frontend/blob/main/CONTRIBUTING.md

[claireashworth]

Closing this issue as the work related to it is occurring as part of the v5 release. specifically issue 2186