Add JAVA_26 to JRE enum

[sormuras]

Overview

Add JRE.JAVA_26 constant and introduce JDK 26-ea CI build

Closes #4642

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes