Support JDK21/gradle 8.4

[slymon99]

Buildship throws errors in JDK21/gradle8.4 (for a standard gradle project that works on JDK17)

!ENTRY org.eclipse.jdt.ls.core 4 2 2023-10-12 13:48:39.495
!MESSAGE Problems occurred when invoking code from plug-in: "org.eclipse.jdt.ls.core".
!STACK 0
org.eclipse.buildship.core.internal.GradlePluginsRuntimeException: Model not available for algo
	at org.eclipse.buildship.core.internal.workspace.GradleClasspathContainerRuntimeClasspathEntryResolver.resolveRuntimeClasspathEntry(GradleClasspathContainerRuntimeClasspathEntryResolver.java:82)
	at org.eclipse.buildship.core.internal.workspace.GradleClasspathContainerRuntimeClasspathEntryResolver.resolveRuntimeClasspathEntry(GradleClasspathContainerRuntimeClasspathEntryResolver.java:58)
	at org.eclipse.jdt.internal.launching.RuntimeClasspathEntryResolver.resolveRuntimeClasspathEntry(RuntimeClasspathEntryResolver.java:49)
	at org.eclipse.jdt.launching.JavaRuntime.resolveRuntimeClasspathEntry(JavaRuntime.java:1261)
	at org.eclipse.buildship.core.internal.launch.GradleClasspathProvider.resolveClasspath(GradleClasspathProvider.java:101)
	at org.eclipse.jdt.internal.launching.RuntimeClasspathProvider.resolveClasspath(RuntimeClasspathProvider.java:68)
	at org.eclipse.jdt.launching.JavaRuntime.resolveRuntimeClasspath(JavaRuntime.java:1663)
	at org.eclipse.jdt.launching.AbstractJavaLaunchConfigurationDelegate.getClasspathAndModulepath(AbstractJavaLaunchConfigurationDelegate.java:464)
	at org.eclipse.jdt.ls.core.internal.commands.ProjectCommand$1.run(ProjectCommand.java:176)
	at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2453)
	at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2478)
	at org.eclipse.jdt.ls.core.internal.commands.ProjectCommand.getClasspathsFromJavaProject(ProjectCommand.java:173)
	at org.eclipse.jdt.ls.core.internal.commands.ProjectCommand.getClasspaths(ProjectCommand.java:153)
	at org.eclipse.jdt.ls.core.internal.JDTDelegateCommandHandler.executeCommand(JDTDelegateCommandHandler.java:95)
	at org.eclipse.jdt.ls.core.internal.handlers.WorkspaceExecuteCommandHandler$1.run(WorkspaceExecuteCommandHandler.java:230)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:45)
	at org.eclipse.jdt.ls.core.internal.handlers.WorkspaceExecuteCommandHandler.executeCommand(WorkspaceExecuteCommandHandler.java:220)
	at org.eclipse.jdt.ls.core.internal.handlers.JDTLanguageServer.lambda$4(JDTLanguageServer.java:614)
	at org.eclipse.jdt.ls.core.internal.BaseJDTLanguageServer.lambda$0(BaseJDTLanguageServer.java:87)
	at java.base/java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:646)
	at java.base/java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:483)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:387)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1312)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1843)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1808)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:188)