Bump com.diffplug.spotless:spotless-plugin-gradle from 6.19.0 to 6.20.0 (open-telemetry#8975)

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

Author: dependabot[bot]

conventions/build.gradle.kts

@@ -54,7 +54,7 @@ dependencies {
   implementation("org.apache.maven:maven-aether-provider:3.3.9")
 
   // When updating, update above in plugins too
-  implementation("com.diffplug.spotless:spotless-plugin-gradle:6.19.0")
+  implementation("com.diffplug.spotless:spotless-plugin-gradle:6.20.0")
   implementation("com.google.guava:guava:32.1.1-jre")
   implementation("gradle.plugin.com.google.protobuf:protobuf-gradle-plugin:0.8.18")
   implementation("com.github.johnrengelman:shadow:8.1.1")