jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to class java.net.URLClassLoader

Hi, we are hitting this error with JDK 11:

```
(Fiji Is Just) ImageJ 2.1.0/1.53c; Java 11.0.11 [64-bit]; Linux 4.15.0-144-generic; 361MB of 79744MB (<1%)

java.lang.RuntimeException: java.util.concurrent.ExecutionException: java.lang.ClassCastException: class jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to class java.net.URLClassLoader (jdk.internal.loader.ClassLoaders$AppClassLoader and java.net.URLClassLoader are in module java.base of loader 'bootstrap')
at net.imagej.legacy.LegacyService.runLegacyCompatibleCommand(LegacyService.java:307)
at net.imagej.legacy.DefaultLegacyHooks.interceptRunPlugIn(DefaultLegacyHooks.java:166)
at ij.IJ.runPlugIn(IJ.java)
at ij.Executer.runCommand(Executer.java:150)
at ij.Executer.run(Executer.java:68)
at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: java.util.concurrent.ExecutionException: java.lang.ClassCastException: class jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to class java.net.URLClassLoader (jdk.internal.loader.ClassLoaders$AppClassLoader and java.net.URLClassLoader are in module java.base of loader 'bootstrap')
at java.base/java.util.concurrent.FutureTask.report(FutureTask.java:122)
at java.base/java.util.concurrent.FutureTask.get(FutureTask.java:191)
at net.imagej.legacy.LegacyService.runLegacyCompatibleCommand(LegacyService.java:303)
... 5 more
Caused by: java.lang.ClassCastException: class jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to class java.net.URLClassLoader (jdk.internal.loader.ClassLoaders$AppClassLoader and java.net.URLClassLoader are in module java.base of loader 'bootstrap')
at net.imagej.tensorflow.util.TensorFlowUtil.getTensorFlowJARVersion(TensorFlowUtil.java:79)
at net.imagej.tensorflow.util.TensorFlowUtil.versionFromClassPathJAR(TensorFlowUtil.java:94)
at net.imagej.tensorflow.ui.TensorFlowLibraryManagementCommand.getTensorFlowJARVersion(TensorFlowLibraryManagementCommand.java:112)
at net.imagej.tensorflow.ui.TensorFlowLibraryManagementCommand.initAvailableVersions(TensorFlowLibraryManagementCommand.java:98)
at net.imagej.tensorflow.ui.TensorFlowLibraryManagementCommand.run(TensorFlowLibraryManagementCommand.java:86)
at org.scijava.command.CommandModule.run(CommandModule.java:196)
at org.scijava.module.ModuleRunner.run(ModuleRunner.java:165)
at org.scijava.module.ModuleRunner.call(ModuleRunner.java:124)
at org.scijava.module.ModuleRunner.call(ModuleRunner.java:63)
at org.scijava.thread.DefaultThreadService.lambda$wrap$2(DefaultThreadService.java:225)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
... 1 more

```

Here is the java version:
```
ac4816754e0b:~/Fiji.app$ java -version
openjdk version "11.0.11" 2021-04-20
OpenJDK Runtime Environment AdoptOpenJDK-11.0.11+9 (build 11.0.11+9)
OpenJDK 64-Bit Server VM AdoptOpenJDK-11.0.11+9 (build 11.0.11+9, mixed mode)
```

cc @esgomezm

Start with net.imagej.tensorflow.util.TensorFlowUtil.java at line 79, then follow versionFromClassPathJAR and TensorFlowLibraryManagementCommand.initAvailableVersions from the reported stack trace. Reproduce the failure with JDK 11 and confirm that the TensorFlow library management command completes without the ClassCastException.

Written by the indexing model from the issue text.