fix: mutated fields cannot be final

[wfouche]

Closes #2417

[wfouche]

Before PR, using Java 26 as default JVM

$ jbang run asciidoctorj@asciidoctor -V
WARNING: Final field catalogRef in class dev.jbang.catalog.CatalogRef has been mutated reflectively by class com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$2 in unnamed module @7bfcd12c (file:/home/wfouche/.sdkman/candidates/jbang/0.137.0/bin/jbang.jar)
WARNING: Use --enable-final-field-mutation=ALL-UNNAMED to avoid a warning
WARNING: Mutating final fields will be blocked in a future release unless final field mutation is enabled
AsciidoctorJ 3.0.1 (Asciidoctor 2.0.26) [https://asciidoctor.org]
Runtime Environment: jruby 9.4.14.0 (3.1.7)

After PR, using Java 26 as default JVM

$ ./build/install/jbang/bin/jbang run asciidoctorj@asciidoctor -V
AsciidoctorJ 3.0.1 (Asciidoctor 2.0.26) [https://asciidoctor.org]
Runtime Environment: jruby 9.4.14.0 (3.1.7)