[release/10.0] [mono][sgen] Fix incorrect condition when checking if we should do a major collection (#120532)

Backport of #120432 to release/10.0

/cc @BrzVlad

## Customer Impact

- [x] Customer reported
- [ ] Found internally

This impacts all platforms using mono. A customer reported a 40x
regression on an XML Serialization benchmark when moving from .NET8 to
.NET9. This regression is caused by the runtime triggering excessive GCs
due to a bug in the heuristic. This is more likely to happen if the GC
concurrent sweep phase takes longer and the application allocates large
objects frequently. This likely impacts many applications, but with less
noticeable performance impact.

## Regression

- [x] Yes
- [ ] No

This regressed in .NET9

## Testing

Tested the fix on sample benchmark that it works correctly.

## Risk

Low risk. The fix is localized and the previous condition was blatantly
wrong by mistake.

Co-authored-by: Vlad Brezae <brezaevlad@gmail.com>

Author: github-actions[bot]

src/mono/mono/sgen/sgen-memory-governor.c

@@ -141,7 +141,7 @@ sgen_need_major_collection_conservative (void)
 	size_t max_allowance = GDOUBLE_TO_SIZE (max_last_collection_heap_size * SGEN_DEFAULT_ALLOWANCE_HEAP_SIZE_RATIO);
 	max_allowance = MAX (max_allowance, GDOUBLE_TO_SIZE (MIN_MINOR_COLLECTION_ALLOWANCE));
 
-	return min_heap_size > max_allowance;
+	return min_heap_size > (max_last_collection_heap_size + max_allowance);
 }
 
 static size_t