Update JDK version from 17 to 21 for extension publishing.

- When rebuilding an extension, that has a language server (that must
  also rebuild), if the language server relies on the latest Eclipse
  Platform, it will have the same BREE minimum
- Eclipse 4.35 (2025-03) requires a JRE 21

Signed-off-by: Roland Grunberg <[email protected]>

Author: rgrunber

.github/workflows/publish-extensions.yml

@@ -48,7 +48,7 @@ jobs:
       - uses: actions/setup-java@v4
         with:
           distribution: "microsoft"
-          java-version: "17"
+          java-version: "21"
       - name: Install dependencies for native modules
         run: |
           sudo apt-get update