build(log4j2): Pin compilation to JDK 8 via Maven Toolchains (#616)

Release 1.6.3 was built on a JDK that does not run annotation processors
by default, which silently dropped the Log4j2 plugin descriptor
(META-INF/.../Log4j2Plugins.dat) for LambdaAppender, LambdaTextFormat,
and LambdaJsonFormat. The published artifact was broken at runtime:
log4j could not resolve <Lambda>, <LambdaTextFormat>, or
<LambdaJsonFormat> elements in user log4j2.xml configurations.

Configure maven-toolchains-plugin to require a JDK 8 toolchain so javac
comes from a JDK that runs annotation processors by default, regardless
of which JVM Maven is invoked under. The version range [1.8,9) matches
both "1.8" and "8".

The existing GitHub Actions workflow at
.github/workflows/aws-lambda-java-log4j2.yml uses actions/setup-java@v5
with java-version: 8 and distribution: corretto. setup-java@v5
auto-generates a ~/.m2/toolchains.xml entry with <version>8</version>,
which the [1.8,9) range matches, so no workflow changes are required.

When no matching JDK 8 toolchain is available, the build now fails fast
at the validate phase with a clear "Cannot find matching toolchain
definitions" error instead of silently producing an artifact missing its
plugin descriptor.

Co-authored-by: Davide Melfi <darklight3it@gmail.com>

Author: rschmitt

aws-lambda-java-log4j2/pom.xml

@@ -79,6 +79,26 @@
         <plugins>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-toolchains-plugin</artifactId>
+                <version>3.2.0</version>
+                <configuration>
+                    <toolchains>
+                        <jdk>
+                            <!-- Range matches both "8" (e.g. actions/setup-java)
+                                 and "1.8" (legacy / hand-written toolchains). -->
+                            <version>[1.8,9)</version>
+                        </jdk>
+                    </toolchains>
+                </configuration>
+                <executions>
+                    <execution>
+                        <goals>
+                            <goal>toolchain</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
+            <plugin>
                 <artifactId>maven-surefire-plugin</artifactId>
                 <version>3.5.2</version>
             </plugin>