Create warning when parsing targets too new Java versions

[mickaelistria]

Fixes #3018

What it does

Create a warning when options specify a new Java source version

How to test

• Create a basic Java Project and a basic Java file in it
• Verify there is no error
• Edit the .settings/org.eclipse.jdt.core.prefs, set the value of org.eclipse.jdt.core.compiler.compliance to 24 (basically last version support by ECJ + 1).
• Reopen the file, edit => a warning is now shown "Java language version '24' is not supported, using version '23' instead"

Author checklist

• I have thoroughly tested my changes
• The change is following the coding conventions
• I have signed the Eclipse Contributor Agreement (ECA)

[mickaelistria]

@jjohnstn @iloveeclipse What do you think?

[jukzi]

How to test

• Create a basic Java Project and a basic Java file in it
• Verify there is no error
• Edit the .settings/org.eclipse.jdt.core.prefs, set the value of org.eclipse.jdt.core.compiler.compliance to 24 (basically last version support by ECJ + 1).
• Reopen the file, edit => a warning is now shown "Java language version '24' is not supported, using version '23' instead"

If the PR would contain a JUnit that demonstrates the desired behavior i would be confident to merge.

[mickaelistria]

If the PR would contain a JUnit that demonstrates the desired behavior i would be confident to merge.

Ack. I'll be back at it shortly.

[mickaelistria]

I added a test case.

[jjohnstn]

LGTM. With the initial review's issue being addressed plus a test case added as requested by @jukzi, I am merging.