In order to allow building tomcat-11.0.6 with java 21 we have to use an older version of eclipse-ecj.
We cannot keyword eclipse-ecj:4.35 as long we don't have Java 23 or higher keyworded.
Otherwise compilation would run into this error:

    [javac] /var/tmp/portage/www-servers/tomcat-11.0.6/work/apache-tomcat-11.0.6-src/java/org/apache/jasper/compiler/JDTCompiler.java:327: error: cannot find symbol
    [javac]                 settings.put(CompilerOptions.OPTION_Source, CompilerOptions.VERSION_23);
    [javac]                                                                            ^
    [javac]   symbol:   variable VERSION_23
    [javac]   location: class CompilerOptions
    [javac] /var/tmp/portage/www-servers/tomcat-11.0.6/work/apache-tomcat-11.0.6-src/java/org/apache/jasper/compiler/JDTCompiler.java:415: error: cannot find symbol
    [javac]                 settings.put(CompilerOptions.OPTION_TargetPlatform, CompilerOptions.VERSION_23);
    [javac]                                                                                    ^
    [javac]   symbol:   variable VERSION_23
    [javac]   location: class CompilerOptions
    [javac] /var/tmp/portage/www-servers/tomcat-11.0.6/work/apache-tomcat-11.0.6-src/java/org/apache/jasper/compiler/JDTCompiler.java:416: error: cannot find symbol
    [javac]                 settings.put(CompilerOptions.OPTION_Compliance, CompilerOptions.VERSION_23);
    [javac]                                                                                ^
    [javac]   symbol:   variable VERSION_23
    [javac]   location: class CompilerOptions

--- a/java/org/apache/jasper/compiler/JDTCompiler.java
+++ b/java/org/apache/jasper/compiler/JDTCompiler.java
@@ -324,7 +324,7 @@ public class JDTCompiler extends org.apache.jasper.compiler.Compiler {
             } else if (opt.equals("22")) {
                 settings.put(CompilerOptions.OPTION_Source, CompilerOptions.VERSION_22);
             } else if (opt.equals("23")) {
-                settings.put(CompilerOptions.OPTION_Source, CompilerOptions.VERSION_23);
+                settings.put(CompilerOptions.OPTION_Source, "23");
             } else if (opt.equals("24")) {
                 // Constant not available in latest ECJ version shipped with
                 // Tomcat. May be supported in a snapshot build.
@@ -412,8 +412,8 @@ public class JDTCompiler extends org.apache.jasper.compiler.Compiler {
                 settings.put(CompilerOptions.OPTION_TargetPlatform, CompilerOptions.VERSION_22);
                 settings.put(CompilerOptions.OPTION_Compliance, CompilerOptions.VERSION_22);
             } else if (opt.equals("23")) {
-                settings.put(CompilerOptions.OPTION_TargetPlatform, CompilerOptions.VERSION_23);
-                settings.put(CompilerOptions.OPTION_Compliance, CompilerOptions.VERSION_23);
+                settings.put(CompilerOptions.OPTION_TargetPlatform, "23");
+                settings.put(CompilerOptions.OPTION_Compliance, "23");
             } else if (opt.equals("24")) {
                 // Constant not available in latest ECJ version shipped with
                 // Tomcat. May be supported in a snapshot build.