Cross referencing my answer on [StackOverflow
As far as I can tell, the Apex Cookie Class does not support the
SameSite attribute at all.
As a result, I would investigate using
httpResponse.setHeader('Set-Cookie', 'cookieLabel=cookieValue; SameSite=None; Secure');
Be aware though, in other frameworks I do see the cookie handling overwrite any existing
Set-Cookie headers so you may want to ensure you do any manual setting of headers either before or after the in-built cookie handling.
I would also raise a feature request for full support of the
SameSite attribute in the framework.