Update website guidance and macro options to deprecate 'Header with service name' variant

[calvin-lau-sig7]

Change current message to 'avoid' using to 'Header with service name' variant to deprecation, and that it will be removed in future release.

Google doc for planning and drafting

What

• Draft guidance changes to help ensure service teams are aware and know what to do.
• Remove/reduce current guidance to do with 'Header with service name' and replace with deprecation messages.
• Encourage use of service navigation component

Includes:

• website guidance
• macro options
• Frontend docs (if needed)

Why

This supports the use of the 'Service navigation' component.

Who needs to work on this

Content designer, technical writer

Who needs to review this

Developers, squad

Done when:

• pages to edit outlined
• drafted content changes
• decide on way to add messages to macro options
• reviewed draft with squad
• added content updates to pull request
• include publish with v5.9.0 release activity

[calvin-lau-sig7]

Looks like we just need to update the trifecta of GOV.UK header, service navigation and navigate a service pattern.

I've drafted needed changes in Google Doc.

It looks pretty basic and clear, I'm slightly mindful this will be a big change that service teams need to be aware of but can see if anything stronger is needed in this edits once we start writing comms.

[calvin-lau-sig7]

Re: adding messages in GOV.UK Frontend code

@domoscargin reckons that there are not any obvious places to edit on Frontend Docs, and no code changes until the things are actually removed in the next breaking release, so nothing more to do on those.

[calvin-lau-sig7]

Start pull request for this work: #4490

Separate PR for macro options: #4488