Bump opentelemetry-instrumentation-bom-alpha in /java (aws-observability#372)

Bumps [opentelemetry-instrumentation-bom-alpha](https://github.com/open-telemetry/opentelemetry-java-instrumentation) from 1.20.2-alpha to 1.21.0-alpha.
- [Release notes](https://github.com/open-telemetry/opentelemetry-java-instrumentation/releases)
- [Changelog](https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/CHANGELOG.md)
- [Commits](https://github.com/open-telemetry/opentelemetry-java-instrumentation/commits)

---
updated-dependencies:
- dependency-name: io.opentelemetry.instrumentation:opentelemetry-instrumentation-bom-alpha
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

java/dependencyManagement/build.gradle.kts

@@ -9,7 +9,7 @@ plugins {
 data class DependencySet(val group: String, val version: String, val modules: List<String>)
 
 val DEPENDENCY_BOMS = listOf(
-    "io.opentelemetry.instrumentation:opentelemetry-instrumentation-bom-alpha:1.20.2-alpha",
+    "io.opentelemetry.instrumentation:opentelemetry-instrumentation-bom-alpha:1.21.0-alpha",
     "org.apache.logging.log4j:log4j-bom:2.19.0",
     "software.amazon.awssdk:bom:2.18.36"
 )