Use Java 26

[cstamas]

Up from 25 to 26.

[cstamas]

@kwin this seems quite consistent: JDK transport UTs fail on Java 26?

[kwin]

Error:  Tests run: 85, Failures: 1, Errors: 0, Skipped: 6, Time elapsed: 4.443 s <<< FAILURE! -- in org.eclipse.aether.transport.jdk.JdkTransporterTest
Error:  org.eclipse.aether.transport.jdk.JdkTransporterTest.testRetryHandler_defaultCount_negative -- Time elapsed: 0.022 s <<< FAILURE!
org.opentest4j.AssertionFailedError: Expected error
	at org.junit.jupiter.api.AssertionUtils.fail(AssertionUtils.java:38)
	at org.junit.jupiter.api.Assertions.fail(Assertions.java:138)
	at org.eclipse.aether.transport.jdk.JdkTransporterTest.testRetryHandler_defaultCount_negative(JdkTransporterTest.java:90)
	at java.base/java.lang.reflect.Method.invoke(Method.java:565)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1612)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1612)

Seems some internal retry mechanism has changed in Java 26...