Dynamically infer support source/target version from chosen CompilationUnitResolver

Currently, JDT uses hardcoded values in ECJ to derive the possible max source/target versions for compilation. However, this static approach tends to require more maintenance and make support for new Java versions harder to unleash, particularly when using an alternative compiler/CompilationUnitResolver.

Instead of using the hardcoded version, one could consider having a method `int lastSupportedCompileVersion()` that would query the chosen compiler. Consumers should be encouraged to use such a method instead of `ClassFileConstants.getLatestJDKLevel()` which is specific to built-in ECJ.

this new method could be kind of implemented as follows (drafting idea, not tested/executable):
```java
public boolean isVersionSupported(int version) {
ASTParser parser = new ASTParser(AST.getJLSLatest());
parser.setCompilerOptions(Map.of(
OPTION_Source, Integer.toString(version),
OPTION_Target, Integer.toString(version)
));
parser.setSource("class Test {}");
var ast = parser.createAST(null);
return Arrays.stream(ast.getProblems()).noneMatch(p -> p.getId() == IProblem.JavaVersionTooRecent);
}

public int lastSupportedCompilerVersion() {
int startVersion = Runtime.getRuntime().version().feature(); // choose some probable enough heuristic
if (isVersionSupported(startVersion)) {
do {
startVersion++;
} while (isVersionSupported(startVersion));
return startVersion - 1;
}
do {
startVersion--;
} while (!isVersionSupported(startVersion));
return startVersion;
}
```