Update CODEOWNERS for the changelog directory (#3777)

## Motivation and Context
Removes `CHANGELOG.next.toml` from `CODEOWNERS` since it no longer
exists. The file now lists the `.changelog` directory instead under
`Shared ownership`.

----

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choice._

CODEOWNERS

@@ -16,9 +16,9 @@
 /rust-runtime/aws-smithy-http-server-typescript/    @smithy-lang/smithy-rs-server
 
 # Shared ownership
+/.changelog/                                        @smithy-lang/aws-sdk-rust @smithy-lang/smithy-rs-server
 /.github/                                           @smithy-lang/aws-sdk-rust @smithy-lang/smithy-rs-server
 /CHANGELOG.md                                       @smithy-lang/aws-sdk-rust @smithy-lang/smithy-rs-server
-/CHANGELOG.next.toml                                @smithy-lang/aws-sdk-rust @smithy-lang/smithy-rs-server
 /README.md                                          @smithy-lang/aws-sdk-rust @smithy-lang/smithy-rs-server
 /build.gradle.kts                                   @smithy-lang/aws-sdk-rust @smithy-lang/smithy-rs-server
 /buildSrc/                                          @smithy-lang/aws-sdk-rust @smithy-lang/smithy-rs-server