Fix sporadic CME in gradle JavaToolchainSupport

[mbien]

• switch to ConcurrentHashMap to avoid computeIfAbsent() throwing CMEs

Noticed it while testing an unrelated PR (#8223 (comment)). Happened only once, either during project creation or project opening (can't remember).

exception

java.util.ConcurrentModificationException
	at java.base/java.util.HashMap.computeIfAbsent(HashMap.java:1230)
	at org.netbeans.modules.gradle.java.spi.support.JavaToolchainSupport.platformByHome(JavaToolchainSupport.java:70)
	at org.netbeans.modules.gradle.java.classpath.BootClassPathImpl.createPath(BootClassPathImpl.java:59)
	at org.netbeans.modules.gradle.java.classpath.AbstractGradleClassPathImpl.getResources(AbstractGradleClassPathImpl.java:108)
	at org.netbeans.api.java.classpath.ClassPath.entries(ClassPath.java:363)
	at org.netbeans.modules.parsing.impl.indexing.RepositoryUpdater.findDependencies(RepositoryUpdater.java:1891)
	at org.netbeans.modules.parsing.impl.indexing.RepositoryUpdater.access$2400(RepositoryUpdater.java:135)
	at org.netbeans.modules.parsing.impl.indexing.RepositoryUpdater$RootsWork.getDone(RepositoryUpdater.java:4965)
	at org.netbeans.modules.parsing.impl.indexing.RepositoryUpdater$InitialRootsWork.getDone(RepositoryUpdater.java:5842)
[catch] at org.netbeans.modules.parsing.impl.indexing.RepositoryUpdater$Work.doTheWork(RepositoryUpdater.java:3452)
	at org.netbeans.modules.parsing.impl.indexing.RepositoryUpdater$Task._run(RepositoryUpdater.java:6197)
	at org.netbeans.modules.parsing.impl.indexing.RepositoryUpdater$Task.access$3400(RepositoryUpdater.java:5855)
	at org.netbeans.modules.parsing.impl.indexing.RepositoryUpdater$Task$2.lambda$call$0(RepositoryUpdater.java:6116)
	at org.openide.util.lookup.Lookups.executeWith(Lookups.java:288)
	at org.netbeans.modules.parsing.impl.RunWhenScanFinishedSupport.performScan(RunWhenScanFinishedSupport.java:83)
	at org.netbeans.modules.parsing.impl.indexing.RepositoryUpdater$Task$2.call(RepositoryUpdater.java:6116)
	at org.netbeans.modules.parsing.impl.indexing.RepositoryUpdater$Task$2.call(RepositoryUpdater.java:6112)
	at org.netbeans.modules.masterfs.filebasedfs.utils.FileChangedManager.priorityIO(FileChangedManager.java:153)
	at org.netbeans.modules.masterfs.providers.ProvidedExtensions.priorityIO(ProvidedExtensions.java:335)
	at org.netbeans.modules.parsing.nb.DataObjectEnvFactory.runPriorityIO(DataObjectEnvFactory.java:118)
	at org.netbeans.modules.parsing.impl.Utilities.runPriorityIO(Utilities.java:67)
	at org.netbeans.modules.parsing.impl.indexing.RepositoryUpdater$Task.run(RepositoryUpdater.java:6112)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:572)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317)
	at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:1403)
	at org.netbeans.modules.openide.util.GlobalLookup.execute(GlobalLookup.java:45)
	at org.openide.util.lookup.Lookups.executeWith(Lookups.java:287)
	at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:2018)

[lkishalmi]

I see nothing wrong with this.

Thanks!