Correctly truncate VM (#9342)

## Problem

https://github.com/neondatabase/neon/issues/9240

## Summary of changes

Correctly truncate VM page instead just replacing it with zero page.

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

---------

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>
Co-authored-by: Heikki Linnakangas <heikki@neon.tech>

libs/pageserver_api/src/record.rs

@@ -41,6 +41,11 @@ pub enum NeonWalRecord {
         file_path: String,
         content: Option<Bytes>,
     },
+    // Truncate visibility map page
+    TruncateVisibilityMap {
+        trunc_byte: usize,
+        trunc_offs: usize,
+    },
 
     /// A testing record for unit testing purposes. It supports append data to an existing image, or clear it.
     #[cfg(feature = "testing")]