Update info about browser support in repo docs

[EoinShaughnessy]

Fixes #2623.

[EoinShaughnessy]

Should we add some detail here about what 'functional' and 'compliant' mean, or would that just repeat what's in Kim's proposal (which we're linking to)?

[EoinShaughnessy]

@vanitabarrett

Line 54 (current version of doc)/line 73 (this PR) also mentions IE8:

[You should also use] a separate stylesheet if you support Internet Explorer 8

Do we need to delete/keep/change this line?

[EoinShaughnessy]

@vanitabarrett Also saw some other IE mentions in the repo...

https://github.com/alphagov/govuk-frontend/blob/main/docs/contributing/resize-text-in-browsers.md#resize-text-in-internet-explorer-versions-8-to-11
Should we leave this as it is for now, in case anyone wants to know how to resize text in IE? If so, should I create an issue so we know to change/remove it when the time comes (i.e., when we've reduced support)?

https://github.com/alphagov/govuk-frontend/blob/main/docs/contributing/coding-standards/css.md#use-single-colons-for-pseudo-element-selectors
Line: "This is to ensure compatibility with Internet Explorer 8, which doesn't support the double colon syntax."
Can we delete this section?

https://github.com/alphagov/govuk-frontend/blob/main/CONTRIBUTING.md#supported-browsers
Line: Your contribution needs to work with certain browsers as set out in README. See also supporting Internet Explorer 8."
Should we delete the second sentence, if the plan is to deprecate the Frontend Docs page about supporting IE8?

[36degrees]

Our approach to browser support has changed since this PR was opened. We'll pick this up again as part of #2718.