I just noticed this alert for a flow in my Dev environment: I am looking at the Details screen of the following flow (which is part of a solution:

Note the warning alert:

Looks like I need to fix this flow. Click on the Flow checker to see the details of the warning:

Flows should use CONNECTION REFERENCES instead of CONNECTIONS. I think this is the issue that needs to be addressed. Old flows are using connections, we need to use CONNECTION REFERENCES. Click on the link "Remove connections so connection references can be used" displayed in the warning:

Click on the flow first step with the warning icon (not the ...), and add New connection reference:

Then for every other step with a connection warnings, perform the above but this time select the new connection reference just created:

Now all the connection references are fixed for this flow:

I just test the flow:

Looking at the solution again, I just noticed that there is a SharePoint connection Reference, which is probably the connection reference used by the flow:

The SharePoint connection reference can be edited, but for now I am going to leave this alone.
Also, note in the blog mentioned previously another method to fix this:
https://www.itaintboring.com/powerapps/connection-references/
"Not sure if there is an easier way to reset the Flow so it starts using connection references, but I just added it to a new solution, exported the solution, deleted both the Flow and my new solution, then imported it back."
Hope this helps.
Ellis