Consolidate changelog entries for once_cell replacement (#4100)

## Motivation and Context
Consolidated changelog entries for #4050 to remove duplicated entries,
and removed it for the tooling directory (which we don't announce it in
release notes since it's an internal change).

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Author: ysaito1001

.changelog/1742160441.md

@@ -4,9 +4,10 @@ applies_to:
 - server
 authors:
 - FalkWoldmann
-references: []
+references:
+- smithy-rs#4050
 breaking: false
 new_feature: false
 bug_fix: false
 ---
-Removes the once_cell crate from the rust-runtime crates
+Replace the `once_cell` crate with the `std` counterpart in Smithy runtime crates.

.changelog/1742673776.md

@@ -3,9 +3,10 @@ applies_to:
 - aws-sdk-rust
 authors:
 - FalkWoldmann
-references: []
+references:
+- smithy-rs#4050
 breaking: false
 new_feature: false
 bug_fix: false
 ---
-Removes the once_cell crate from the `rust-runtime` crate
+Replace the `once_cell` crate with the `std` counterpart in AWS runtime crates.