How can I resolve a cross-site Google Analytics cookie `SameSite=None` warning in Chrome on Apache 2.4 and PHP 7.1?

I got a response from Google Chrome Labs after I posted a similar question on their github page.

The cookies triggering the warning are coming from google.com so you will not be able to alter them. The Ads team is aware of these issues and is working to get their cookies fixed before the Feb 2020 stable date. It also means that none of the header directives you're specifying will affect the google.com cookie, it will only cover cookies set for your site.

If you have any cookie warnings that specifically list a domain you control, then you will need to add the correct attributes. -rowan-m

I would look at the tracker script. Here is the section about cross-domain traffic in the gtag.js docs. Make sure only the domain is present and no www, http, ect.

gtag('set', 'linker', {
  'domains': ['example.com', 'example-b.com']
});