Bump io.opentelemetry.instrumentation:gradle-plugins

Bumps [io.opentelemetry.instrumentation:gradle-plugins](https://github.com/open-telemetry/opentelemetry-java-instrumentation) from 2.19.0-alpha to 2.21.0-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:gradle-plugins
  dependency-version: 2.21.0-alpha
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

buildSrc/build.gradle.kts

@@ -36,7 +36,7 @@ dependencies {
 
   implementation("org.owasp:dependency-check-gradle:12.1.8")
 
-  implementation("io.opentelemetry.instrumentation:gradle-plugins:2.19.0-alpha")
+  implementation("io.opentelemetry.instrumentation:gradle-plugins:2.21.0-alpha")
 
   implementation("net.ltgt.gradle:gradle-errorprone-plugin:4.3.0")
   implementation("net.ltgt.gradle:gradle-nullaway-plugin:2.3.0")