Add support for Java 21

CHANGES.txt

@@ -188,6 +188,7 @@ RELEASE  VERSION/DATE TO BE FILLED IN LATER
       avoid the GNU linker issuing warnings for its absence.
     - Eliminate more http: references (mostly in comments/docstrings where
       they really weren't harmful). A few links labeled dead with no alt.
+    - Add JDK 21 LTS support
 
 
   From Jonathon Reinhart:

RELEASE.txt

@@ -78,6 +78,7 @@ CHANGED/ENHANCED EXISTING FUNCTIONALITY
   although the preferred usage remains to import from SCons.Util only.
   Any code that did the direct import will have to change to import from
   SCons.Util.sctypes.
+- Add JDK 21 LTS support
 
 FIXES
 -----

SCons/Tool/JavaCommon.py

@@ -124,6 +124,7 @@ def __init__(self, version=default_java_version) -> None:
                 '18.0',
                 '19.0',
                 '20.0',
+                '21.0',
             ):
                 msg = "Java version %s not supported" % version
                 raise NotImplementedError(msg)
@@ -251,6 +252,7 @@ def addAnonClass(self) -> None:
                 '18.0',
                 '19.0',
                 '20.0',
+                '21.0',
             ):
                 self.stackAnonClassBrackets.append(self.brackets)
                 className = []