etcdutl: Implement migrate --force command

Co-authored-by: nic-chen <33000667+nic-chen@users.noreply.github.com>
Co-authored-by: Heisenberg <yuzhiquanlong@gmail.com>

server/storage/schema/version.go

@@ -59,3 +59,8 @@ func UnsafeSetStorageVersion(tx backend.BatchTx, v *semver.Version) {
 	sv := semver.Version{Major: v.Major, Minor: v.Minor}
 	tx.UnsafePut(Meta, MetaStorageVersionName, []byte(sv.String()))
 }
+
+// UnsafeClearStorageVersion removes etcd storage version in backend.
+func UnsafeClearStorageVersion(tx backend.BatchTx) {
+	tx.UnsafeDelete(Meta, MetaStorageVersionName)
+}