GH-1503: update everything to Java 21

* Switch to JDT LS 1.45 and Java 21
* Java 21 for LS side
* Switch to Java 21 the branding plugin

---------

Signed-off-by: Martin Lippert <martin.lippert@broadcom.com>
Co-authored-by: Alex Boyko <alex.boyko@broadcom.com>

eclipse-extensions/org.springframework.ide.eclipse.boot.dash/META-INF/MANIFEST.MF

@@ -46,7 +46,7 @@ Require-Bundle: org.eclipse.ui,
  org.eclipse.lsp4j,
  org.eclipse.jdt.debug,
  org.springsource.ide.eclipse.commons.boot.ls
-Bundle-RequiredExecutionEnvironment: JavaSE-17
+Bundle-RequiredExecutionEnvironment: JavaSE-21
 Bundle-ActivationPolicy: lazy
 Bundle-ClassPath: .
 Export-Package: org.springframework.ide.eclipse.boot.dash,