Require Java 21 for the Eclipse plugin and fix its install steps

Author: abashev

open-java-format-eclipse-plugin/README.md

@@ -3,18 +3,19 @@
 ## Installation
 
 1. Run `./gradlew :open-java-format-eclipse-plugin:build` in the main folder,
-1. If running Eclipse under JRE 17 or later add these options to `eclipse.ini` after `-vmargs`:
+1. Eclipse has to run on Java 21 or later, as current Eclipse packages do. The formatter reaches into
+   javac, so add these options to `eclipse.ini` after `-vmargs`:
    ```
-   --add-exports
-   jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED
-   --add-exports
-   jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED
-   --add-exports
-   jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED
-   --add-exports
-   jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED
-   --add-exports
-   jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED
+   --add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED
+   --add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED
+   --add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED
+   --add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED
+   --add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED
    ```
+   Keep each option and its value on one line, joined by `=`: the Eclipse launcher starts the JVM
+   inside its own process, and an option split over two lines does not take effect — formatting then
+   fails with an `IllegalAccessError` in the workspace log.
 1. Copy `open-java-format-eclipse-plugin/build/libs/open-java-format-eclipse-plugin-<version>.jar` to the `dropins` folder of your Eclipse installation,
-1. Run `eclipse -clean`.
+1. Run `eclipse -clean`,
+1. Pick `open-java-format` in Window → Preferences (Eclipse → Settings on macOS) → Java → Code Style →
+   Formatter → Formatter implementation.

open-java-format-eclipse-plugin/src/main/resources/META-INF/MANIFEST.MF

@@ -2,7 +2,7 @@ Manifest-Version: 1.0
 Bundle-ManifestVersion: 2
 Bundle-Name: open-java-format
 Bundle-SymbolicName: open-java-format-eclipse-plugin;singleton:=true
-Bundle-RequiredExecutionEnvironment: JavaSE-11
+Bundle-RequiredExecutionEnvironment: JavaSE-21
 Import-Package: org.eclipse.jdt.core.dom,
  org.eclipse.jdt.core.formatter,
  org.eclipse.jface.text,