Packages might need to be updated. Try create a brand-new project in 2021.2.0 and see if the Packages/manifest.json is substantively different.
Sometimes the package system gets borked from all this unnecessary churn and requires the package cache to be cleared:
Also, this: extra unwanted packages in new projects (collab, testing, rider and other junk):
About the fastest way I have found to make a project and avoid all this noise is to create the project, then as soon as you see the files appear, FORCE-STOP (hard-kill) Unity (with the Activity Manager or Task Manager), then go hand-edit the Packages/manifest.json file as outlined in the above post, then reopen Unity.