config: rename skip-sql-invoice-migration to skip-native-sql-migration

In future releases we will use this flag to make potentially failing kv
to native SQL migrations optional. For this reason it is better to
rename it to avoid confusion.

sample-lnd.conf

@@ -1478,9 +1478,10 @@
 ; own risk.
 ; db.use-native-sql=false
 
-; If set to true, native SQL invoice migration will be skipped. Note that this
-; option is intended for users who experience non-resolvable migration errors.
-; db.skip-sql-invoice-migration=false
+; If set to true, the KV to native SQL migration will be skipped. Note that
+; this option is intended for users who experience non-resolvable migration
+; errors.
+; db.skip-native-sql-migration=false
 
 [etcd]