Update docs to identify compatibility mode information and associated variables, mixins and settings as unavailable from 5.0

[owenatgov]

What

Identify settings, variables and any other content pertaining to compatibility mode and the support of legacy design systems and identify it as deprecated and no longer available in 5.0.

Why

We are removing compatibility mode in 5.0 and should reflect this in our documentation.

Part of alphagov/govuk-frontend#2769

Who needs to work on this

Developers, Tech writers

Done when

• Identify settings, variables and mixins notes in the epic from the Sass API reference and note that they aren't available in 5.0
• Add a note to the Use with our old frameworks or colours page that we don't support this in 5.0
• Add a note to the Migrate from our old frameworks page that we no longer support compatibility mode as of 5.0

[owenatgov]

We think this work could be investigated and completed as part of alphagov/govuk-frontend#3483 as compatibility mode and the docs surrounding it are an explicit feature that will be in v4 but won't be in v5.

[36degrees]

@owenatgov we've now released v5 – is this still useful in terms of communicating the change? If not, can we close this (and the epic?)

[owenatgov]

We can indeed close this. This was superseded by work to split the v4 and v5 docs, which is now done.