fix(build): Move --add-opens option to Maven profile

The `--add-opens` JVM option is now included in a dedicated Maven profile to avoid applying it during CI builds on JRE 8, where it is unsupported and unnecessary.

Author: ppkarwasz

log4j-to-slf4j/pom.xml

@@ -154,16 +154,44 @@
         </executions>
       </plugin>
 
-      <!-- Illegal access is disabled by default in Java 16 due to JEP-396.
-           We are relaxing it for tests. -->
-      <plugin>
-        <groupId>org.apache.maven.plugins</groupId>
-        <artifactId>maven-surefire-plugin</artifactId>
-        <configuration>
-          <argLine>--add-opens java.base/java.lang=ALL-UNNAMED</argLine>
-        </configuration>
-      </plugin>
-
     </plugins>
   </build>
+
+  <profiles>
+
+    <!-- Fixes incompatible with Java 8 -->
+    <profile>
+
+      <id>java8-incompat-fixes</id>
+
+      <!-- CI uses Java 8 for running tests.
+         Hence, we assume CI=Java8 and apply our changes elsewhere.
+
+         One might think why not activate using `<jdk>[16,)` instead?
+         This doesn't work, since the match is not against "the JDK running tests", but "the JDK running Maven".
+         These two JDKs can differ due to Maven Toolchains.
+         See `java8-tests` profile in `/pom.xml` for details. -->
+      <activation>
+        <property>
+          <name>!env.CI</name>
+        </property>
+      </activation>
+
+      <!-- Illegal access is disabled by default in Java 16 due to JEP-396.
+         We are relaxing it for tests. -->
+      <build>
+        <plugins>
+          <plugin>
+            <groupId>org.apache.maven.plugins</groupId>
+            <artifactId>maven-surefire-plugin</artifactId>
+            <configuration>
+              <argLine>--add-opens java.base/java.lang=ALL-UNNAMED</argLine>
+            </configuration>
+          </plugin>
+        </plugins>
+      </build>
+
+    </profile>
+
+  </profiles>
 </project>