Bump checkstyle from 8.14 to 8.15

[dependabot-preview]

Bumps checkstyle from 8.14 to 8.15.

Release notes

Sourced from checkstyle's releases.

checkstyle-8.15

https://checkstyle.org/releasenotes.html#Release_8.15

Commits

• 67db865 [maven-release-plugin] prepare release checkstyle-8.15
• 12b58ab Issue #5601: remove deploy to sourceforge.net from release.sh
• 585c236 doc: release notes for 8.15
• ba435b8 Issue #6096: improve performance with not lazy usage of file.lastModified()
• 2b1f711 minor: exclude google style files from validation at all
• 577c23d minor: move english.words and words.rpm file to .ci-temp folder and exclude i...
• 3aaa1b6 config: bump guava to 27.0.1-jre
• 8cc835d config: bump system-rules to 1.19.0
• 76f5c17 Revert "config: fix wercker CI, see details at #6218"
• 5b09f16 config: forced more travis CI to run for all file changes
• Additional commits viewable in compare view

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