Bump gradle-wrapper from 5.4 to 5.6.4

* Bump gradle-wrapper from 5.4 to 5.6.4

Bumps [gradle-wrapper](https://github.com/gradle/gradle) from 5.4 to 5.6.4.
- [Release notes](https://github.com/gradle/gradle/releases)
- [Commits](gradle/gradle@v5.4.0...v5.6.4)

---
updated-dependencies:
- dependency-name: gradle-wrapper
  dependency-version: 5.6.4
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

* Fix gradle enableFeaturePreview deprecation warning

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Robert Schlabbach <robert.schlabbach@ubitricity.com>

Author: dependabot[bot]

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-5.4-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-5.6.4-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists

settings.gradle

@@ -4,7 +4,11 @@
 
 rootProject.name = 'java-oca-ocpp'
 
-enableFeaturePreview('STABLE_PUBLISHING')
+import org.gradle.util.GradleVersion
+
+if (GradleVersion.current() < GradleVersion.version("5.0")) {
+    enableFeaturePreview('STABLE_PUBLISHING')
+}
 
 include(':ocpp16')
 include(':ocpp16j')