fix(spring): Support Spring Boot 4.1 (#5573)

* fix(spring): Support Spring Boot 4.1

Bump OpenTelemetry and Spring Boot dependencies to compatible versions so the Spring Boot 4 OpenTelemetry sample works with Spring Boot 4.1.

Keep the Spring 7 sample on the repository Kotlin compiler version when importing the Spring Boot BOM, and add Spring Boot 4.1 to the system test matrix.

Fixes GH-5561

Co-Authored-By: Claude <noreply@anthropic.com>

* changelog

* build: Bump Kotlin to 2.3.21

Align the global Kotlin version with Spring Boot 4.1 and remove the now-unneeded Spring 7 BOM override.

Also remove unused Spring 7-specific Kotlin aliases from the version catalog.

Co-Authored-By: Claude <noreply@anthropic.com>

* docs(changelog): Move bump entry to Unreleased and add version details

Move the Spring Boot 4 / OpenTelemetry dependency bump entry from
8.44.1 to the Unreleased section and add sub-list items showing
each dependency version change.

Co-Authored-By: Claude <noreply@anthropic.com>

* fix(spring): Fix Spring 7 sample test compilation in SB4 matrix

The Spring 7 sample had two dependency issues causing
compileTestKotlin to fail in the Spring Boot 4.x CI matrix:

1. Used libs.springboot.starter.test (Spring Boot 2.7.18) instead of
   libs.springboot4.starter.test, mixing Spring Framework 5.x test
   utilities with a Spring Framework 7.x runtime.

2. Missing Kotlin BOM import in dependencyManagement. Without it,
   Spring Boot 4.0.0's BOM downgraded kotlin-stdlib to 2.2.21 while
   the compiler and build tools remained at 2.3.21, causing
   ClasspathEntrySnapshotTransform initialization failures.

Co-Authored-By: Claude <noreply@anthropic.com>

* mention BOM in changelog

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Co-authored-by: Claude <noreply@anthropic.com>