Update to Gradle 6.7.1 and convetions plugins.

build.gradle

buildSrc/.gitignore

buildSrc/build.gradle

...ain/groovy/relast2uml.java-application-conventions.gradle

...src/main/groovy/relast2uml.java-common-conventions.gradle

...rc/main/groovy/relast2uml.java-jastadd-conventions.gradle

dumpAst/build.gradle

dumpAstWithPlantuml/build.gradle

gradle/wrapper/gradle-wrapper.jar

gradle/wrapper/gradle-wrapper.properties

gradlew

gradlew.bat

grammar2uml/build.gradle

testDumper/build.gradle