Bump com.gradle.plugin-publish from 0.19.0 to 0.21.0

Bumps com.gradle.plugin-publish from 0.19.0 to 0.21.0.

---
updated-dependencies:
- dependency-name: com.gradle.plugin-publish
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

build.gradle

@@ -11,7 +11,7 @@ plugins {
     id 'maven-publish'
     id 'idea'
     id 'jacoco'
-    id 'com.gradle.plugin-publish' version '0.19.0'
+    id 'com.gradle.plugin-publish' version '0.21.0'
     id 'com.github.kt3k.coveralls' version '2.12.0'
 }