Groovy: guard calls to isSealed() with the check for Groovy 4

[greg-at-moderne]

What's changed?

• Adding a guard against usage of isSealed() which is a new method in Groovy 4, it won't work in Groovy 2.
• Fixing the groovy2Test to run using Groovy 2 again, this got likely accidentally broken with Use Groovy 4 parser #6228.

What's your motivation?

Fix:

Caused by: java.lang.NoSuchMethodError: org.codehaus.groovy.ast.ClassNode.isSealed()Z
    at org.openrewrite.groovy.GroovyParserVisitor$RewriteGroovyClassVisitor.visitClass(GroovyParserVisitor.java:331)
    at org.openrewrite.groovy.GroovyParserVisitor.convertTopLevelStatement(GroovyParserVisitor.java:2647)
    at org.openrewrite.groovy.GroovyParserVisitor.visit(GroovyParserVisitor.java:196)

caught by https://github.com/openrewrite/rewrite-gradle-plugin