Modernize and improve module jdk.sctp

During the fix of https://bugs.openjdk.org/browse/JDK-8296024, several improvement areas were identified for the module `jdk.sctp` including:

* Replacing duplicate code segments
* Making certain fields final
* Using enhanced switch
* Using records
* Fixing typos
* Marking fields participating in serialisation with `@Serial`
* Modernizing toString() implementations
* Using pattern matching
* Using diamond operators