pageserver: demote range end fallback log

[VladLazar]

Problem

This trace is emitted whenever a vectored read touches the end of a delta layer file. It's a perfectly normal case, but I expected it to be more rare when implementing the code.

Summary of changes

Demote log to debug.

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

[github-actions]

2760 tests run: 2642 passed, 0 failed, 118 skipped (full report)

---

Code coverage* (full report)

• functions: 27.9% (6395 of 22950 functions)
• lines: 46.4% (44730 of 96488 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
55e8ef4 at 2024-04-17T10:23:26.948Z :recycle:}