Currently backup cursors can only be opened when WiredTiger is in a non-live restore mode. This ticket is to update that logic to also allow backup cursors to open when live restore is enabled and we are in the complete stage. All other stages should still disallow live restore.

We need to make code changes in 3 locations:

1. The live restore check in __session_open_cursor_int
2. Updating the assert in [__wt_live_restore_clean_metadata_string|https://github.com/wiredtiger/wiredtiger/pull/11651/files#diff-c9c0b2a6af0578015371199d5db26e184289da4f9f92215210cc7b04e188ecc1R1243] to also allow the complete phase
3. Updating live_restore06 to also test taking backups when live restore is enabled and in the complete phase. Currently it requires wiredtiger to restart in non-live restore mode before taking the backup