Provide a better fallback for running without a security manager for groovysh on JDK18

When running groovysh on JDK18 and above, the use of the security manager to prohibit calling System.exit() needs to be explicitly enabled by setting java.security.manager to allow.

Previously, when setting that property, groovysh runs (with an expected WARNING) and guards against attempts to call System.exit(). This behavior is unchanged by this issue.

Previously, when that property is not set, groovysh won't start and instead fails with an UnsupportedOperationException. This issue changes the behavior when the property is not set to not attempt to use the NoExitSecurityManager. Instead, groovysh starts but attempts to call System.exit() cause groovysh to immediately exit.