chore(deps): bump com.github.spotbugs from 6.0.20 to 6.0.21

Bumps com.github.spotbugs from 6.0.20 to 6.0.21.

---
updated-dependencies:
- dependency-name: com.github.spotbugs
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

build.gradle

@@ -5,7 +5,7 @@ plugins {
  id 'maven-publish'
  id 'signing'
  id 'jacoco'
- id 'com.github.spotbugs' version '6.0.20'
+ id 'com.github.spotbugs' version '6.0.21'
  id "org.sonarqube" version "5.1.0.4882"
  id 'io.github.gradle-nexus.publish-plugin' version '2.0.0'
 }