Add preview support for Java 17

[basil]

As of jenkinsci/jenkins#6356 there is no longer any need for --add-opens on Java 17, but there is still a problem with the version check in this repository which causes a hard failure on anything other than Java 8 and Java 11. I considered removing this entirely, as it seems to be a bit heavy-handed, but then I decided that it is a useful guardrail to prevent users from accidentally running Jenkins in an unsupported configuration. So instead I am just skipping the Java version check when the --enable-future-java Jenkins option is provided, which is needed to enable Java 17 preview support anyway.

Testing done

Functional testing: started Jenkins successfully with Java 17 and this systemd configuration (with jenkinsci/jenkins#6356):

[Service]
Environment="JAVA_HOME=/usr/lib/jvm/java-17-openjdk-amd64"
Environment="JENKINS_OPTS=--enable-future-java"
Environment="JENKINS_WAR=/home/basil/src/jenkinsci/jenkins/war/target/jenkins.war"

Regression testing: started Jenkins successfully with Java 11 and this systemd configuration:

[Service]
Environment="JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64"

Fixes #217

[basil]

Also tested starting with Java 17 without --enable-future-java and confirmed that it still correctly fails. While testing that, found a minor bug in the display of the error message which I corrected in commit af308c4.