Add JDK 21 (#5498)

https://jdk.java.net/21

For compiler-explorer/infra#1094.

etc/config/java.amazon.properties

@@ -1,7 +1,7 @@
-compilers=&java11:&java12:&java13:&java14:&java15:&java16:&java17:&java18:&java19:&java20
+compilers=&java11:&java12:&java13:&java14:&java15:&java16:&java17:&java18:&java19:&java20:&java21
 compilerType=java
 versionFlag=-version
-defaultCompiler=java2000
+defaultCompiler=java2100
 demangler=
 postProcess=
 options=
@@ -84,3 +84,10 @@ group.java20.objdumper=/opt/compiler-explorer/jdk-20.0.0/bin/javap
 compiler.java2000.exe=/opt/compiler-explorer/jdk-20.0.0/bin/javac
 compiler.java2000.runtime=/opt/compiler-explorer/jdk-20.0.0/bin/java
 compiler.java2000.semver=20.0.0
+
+group.java21.compilers=java2100
+group.java21.groupName=OpenJDK 21
+group.java21.objdumper=/opt/compiler-explorer/jdk-21.0.0/bin/javap
+compiler.java2100.exe=/opt/compiler-explorer/jdk-21.0.0/bin/javac
+compiler.java2100.runtime=/opt/compiler-explorer/jdk-21.0.0/bin/java
+compiler.java2100.semver=21.0.0