[java] Update eclipse-temurin Docker tag to v21 #524

Job	Run time
Find all Bootstraps with a devcontainer spec	6s
Validate Bootstrap clojure	29s
Validate Bootstrap csharp	47s
Validate Bootstrap denojs	38s
Validate Bootstrap elixir	1m 26s
Validate Bootstrap elm	37s
Validate Bootstrap fsharp	55s
Validate Bootstrap gleam	57s
Validate Bootstrap golang	1m 4s
Validate Bootstrap haskell	2m 8s
Validate Bootstrap java	37s
Validate Bootstrap kotlin	1m 10s
Validate Bootstrap nodejs	50s
Validate Bootstrap php	46s
Validate Bootstrap purescript	1m 27s
Validate Bootstrap python	40s
Validate Bootstrap rescript	57s
Validate Bootstrap ruby	44s
Validate Bootstrap rust	55s
Validate Bootstrap scala2	56s
Validate Bootstrap scala3	1m 7s
Validate Bootstrap typescript	35s
	19m 51s

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