Support java 21

[chenrui333]

Consider java 21 has been released for quite some time, it would be good to get it supported.

I just did some gradle run, and ran into some issue against 1.3.9 release:

==> /opt/homebrew/Cellar/kotlin-language-server/1.3.9/bin/kotlin-language-server
Picked up _JAVA_OPTIONS: -Duser.home=/Users/rui/Library/Caches/Homebrew/java_cache -Djava.io.tmpdir=/private/tmp
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
Jan 19, 2024 4:07:16 PM org.eclipse.lsp4j.jsonrpc.services.GenericEndpoint lambda$recursiveFindRpcMethods$2
SEVERE: A delegate object is null, jsonrpc methods of 'public default org.eclipse.lsp4j.services.NotebookDocumentService org.eclipse.lsp4j.services.LanguageServer.getNotebookDocumentService()' are ignored

[fwcd]

The SLF4J message is expected (unfortunately) and the severe error should be fixed upstream with eclipse-lsp4j/lsp4j#659. We should update LSP4J anyway.