From the release notes:
A new Manifest.toml format is now used that captures extensible metadata fields, including the julia version that generated the manifest. Old format manifests are still supported and will be maintained in their original format, unless the user runs Pkg.upgrade_manifest() to upgrade the format of the current environment’s manifest without re-resolving (#40765).
Nothing is wrong, you’re just getting a warning so you know there’s been a change of versions. You can run upgrade_manifest to get rid of the warning.