Thank you @xabbuh.

is symfony/serializer 6.3 actually compatible with property-info 6.4 ? We might have added this conflict for a reason.

I think the reason was being overzealous. I tried locally, seems to work. Lets see the CI.

Well, as you haven't change the dev requirement, the CI will still not try to use older versions.

That's not the point: the point is to allow Serializer 5.4 to use PropertyInfo 6.4, which does work (verified by the CI).
That's what I mean by being overzealous: the conflict was added in good faith to match what require-dev says, but this is actually too restrictive, and excludes newer versions for no valid reasons for users of older stacks.

@stof not updating the constraint in require-dev was a mistake, #59010 fixed that