Improve Maven configuration and document known Java/Maven warnings

- Update compiler configuration to use --release 21 instead of source/target
- Fixes 'location of system modules not set' warnings
- Add Known Issues section to README documenting harmless sun.misc.Unsafe warnings
- These warnings come from Maven's bundled Guice, not from our library code
- Warnings are harmless and will be resolved when Maven updates Guice version

Author: wassertim

README.md

@@ -125,4 +125,16 @@ For each `@DynamoMappable` class, the processor generates:
 - **Bidirectional methods**: `toDynamoDbAttributeValue()` and `fromDynamoDbAttributeValue()`
 - **Dependency injection**: Constructor injection for required mappers
 - **Null safety**: Proper null handling throughout mapping logic
-- **Error handling**: Safe parsing with fallback to null values
+- **Error handling**: Safe parsing with fallback to null values
+
+## Known Issues
+
+### Java/Maven Compatibility Warnings
+
+When using Java 21+ with Maven, you may see warnings like:
+```
+WARNING: A terminally deprecated method in sun.misc.Unsafe has been called
+WARNING: sun.misc.Unsafe::staticFieldBase has been called by com.google.inject.internal.aop.HiddenClassDefiner
+```
+
+These warnings are **harmless** and come from Google Guice (bundled with Maven), not from this library. They will be resolved when Maven updates to a newer Guice version. The warnings do not affect functionality and can be safely ignored.

integration-tests/pom.xml

@@ -78,8 +78,7 @@
                 <artifactId>maven-compiler-plugin</artifactId>
                 <version>3.14.0</version>
                 <configuration>
-                    <source>${maven.compiler.source}</source>
-                    <target>${maven.compiler.target}</target>
+                    <release>21</release>
                     <encoding>${project.build.sourceEncoding}</encoding>
                     <!-- Re-enable DynamoDB Toolkit annotation processor -->
                     <annotationProcessorPaths>

pom.xml

@@ -70,8 +70,7 @@
                 <artifactId>maven-compiler-plugin</artifactId>
                 <version>3.14.0</version>
                 <configuration>
-                    <source>${maven.compiler.source}</source>
-                    <target>${maven.compiler.target}</target>
+                    <release>21</release>
                     <encoding>${project.build.sourceEncoding}</encoding>
                     <annotationProcessorPaths>
                         <path>