Bump checkstyle from 8.25 to 8.26

Bumps [checkstyle](https://github.com/checkstyle/checkstyle) from 8.25 to 8.26.
- [Release notes](https://github.com/checkstyle/checkstyle/releases)
- [Commits](checkstyle/checkstyle@checkstyle-8.25...checkstyle-8.26)

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

build.gradle

@@ -215,7 +215,7 @@ dependencies {
  testCompile "org.testfx:testfx-core:4.0.15-alpha"
  testCompile "org.testfx:testfx-junit5:4.0.15-alpha"
 
- checkstyle 'com.puppycrawl.tools:checkstyle:8.25'
+ checkstyle 'com.puppycrawl.tools:checkstyle:8.26'
  xjc group: 'org.glassfish.jaxb', name: 'jaxb-xjc', version: '2.3.2'
  jython 'org.python:jython-standalone:2.7.1'
 }