SecurityManager is deprecated for removal

[cushon]

The SecurityManager API is deprecated for removal, see https://openjdk.org/jeps/486

It is being used for getClassContext:

slf4j/slf4j-api/src/main/java/org/slf4j/helpers/Util.java

Lines 64 to 66 in 0def25e

	private static final class ClassContextSecurityManager extends SecurityManager {
	protected Class<?>[] getClassContext() {
	return super.getClassContext();

That use-case could be migrated to java.lang.StackWalker API added in Java 9: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/StackWalker.html

There was a draft PR to migrate to StackWalker in #271, and there was a related previous issue in https://jira.qos.ch/browse/SLF4J-528