Bump the `recommended-java-version` to 21 #39580

Job	Run time
Attach pull request number	2s
Initial JDK 17 Build	12m 48s
CI Sanity Check	31s
Calculate Test Jobs	15s
JVM Tests - JDK 17	1h 56m 54s
Gradle Tests - JDK 17	24m 37s
JVM Tests - JDK 21	4h 16m 40s
Devtools Tests - JDK 17	11m 35s
Gradle Tests - JDK 17 Windows	43m 29s
Maven Tests - JDK 17	57m 10s
Devtools Tests - JDK 21	11m 45s
JVM Tests - JDK 17 Windows	4h 17m 48s
Devtools Tests - JDK 17 Windows	18m 22s
Maven Tests - JDK 17 Windows	1h 14m 44s
Kubernetes Tests - JDK 17	12m 35s
Quickstarts Compilation - JDK 17	6m 27s
Kubernetes Tests - JDK 21	12m 13s
MicroProfile TCKs Tests	5m 7s
Native Tests - Main	7m 25s
Kubernetes Tests - JDK 17 Windows	18m 3s
Native Tests - Virtual Thread - Main	22m 46s
Native Tests - Data1	31m 56s
Native Tests - Virtual Thread - Messaging	9m 57s
Native Tests - Data2	27m 12s
Native Tests - Data3	24m 44s
Native Tests - Data4	18m 25s
Native Tests - Data5	21m 26s
Native Tests - Data6	29m 50s
Native Tests - Data7	36m 58s
Native Tests - Amazon	16m 13s
Native Tests - Messaging1	32m 5s
Native Tests - Messaging2	13m 54s
Native Tests - Security1	22m 36s
Native Tests - Security2	28m 26s
Native Tests - Security3	14m 10s
Native Tests - Cache	15m 41s
Native Tests - HTTP	40m 2s
Native Tests - Misc1	27m 8s
Native Tests - Misc2	15m 49s
Native Tests - Misc3	29m 21s
Native Tests - Misc4	44m 22s
Native Tests - Spring	21m 23s
Native Tests - gRPC	20m 40s
Native Tests - Windows - RESTEasy Jackson	5m 42s
Native Tests - DevTools Integration Tests	3m 52s
Native Tests - AWT, ImageIO and Java2D	7m 27s
Build report	43s
	1d 1h 31m 18s

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