Gradle fixes (#621)

* Set up max heap size and max metadata size for root project

We use the root project with Github Actions workflows.
However, the max metadata and the max heap size
are only set up in ./skiko subproject,
so they are not picked up in Github workflows,
which may lead to tests crashing on Linux

* Update Gradle for root and ./skiko

gradle.properties

+org.gradle.jvmargs=-Xmx3G -XX:MaxMetaspaceSize=512m

gradle/wrapper/gradle-wrapper.properties

 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-7.2-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-7.5.1-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists