Adding "min" score mode to parent-child queries

Support for "max", "sum", and "avg" already existed.

Closes #7603 
Wonder if it would be better/safer to be

```java
assert doc.docId >= 0 && doc.docId != NO_MORE_DOCS;
```

If it's unrealistic that `NO_MORE_DOCS` will change, then just making it `&&` is probably enough, but I feel like `Integer.MIN_VALUE` would have probably been a better value.