Document fallback image change to header in 4.0 release notes

[hannalaakso]

What

We made a markup change to the header in #2229 to fix a blank data URI from triggering a CSP error.

We would have initially considered this work to form part of #2340, but we should actually considering documenting this in our release notes for 4.0 as a breaking change for users of the HTML version of the header.

Why

The reason it should be fast tracked ahead of #2340 is that:

• it fixes a potential CSP error
• the <image> tag which the change removes is being flagged by markup validators as deprecated

Who needs to know about this

Developers; Tech Writer

Done when

• We have added the markup change to the 4.0 release notes as a breaking change for users of the HTML version of the header component

[36degrees]

Having caught up with Eoin, I'm going to close this as it's very likely that we're going to need to ask users to make changes to the header markup as a result of #1739. If we included both things as discrete entries in the release notes, both would touch the same lines of code and would need to be made in a certain order.

Instead, I'll add a note to that issue to make sure that we consider this when writing the release notes for that – ensuring that the release notes work whether users have made the changes in #2229 or not, and that we prompt them to do so if not.