Bump com.diffplug.spotless from 8.4.0 to 8.6.0 in /usage-demo

Bumps com.diffplug.spotless from 8.4.0 to 8.6.0.

---
updated-dependencies:
- dependency-name: com.diffplug.spotless
  dependency-version: 8.6.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

usage-demo/build.gradle

@@ -2,7 +2,7 @@
 
 plugins {
     id 'java'
-    id 'com.diffplug.spotless' version '8.4.0'
+    id 'com.diffplug.spotless' version '8.6.0'
     id 'net.ltgt.errorprone' version '5.1.0'
     id 'org.checkerframework' version '1.0.2' apply false
 }