modernise code based on Java 17 norms

[pjfanning]

Pattern matching instanceof — 22 edits in 13 files

• Eliminated verbose instanceof + cast patterns across XmlAnnotationIntrospector, XmlFactory, XmlMapper, StaxUtil, AnnotationUtil, Stax2JacksonReaderAdapter, XmlRootNameLookup, XmlReadContext, XmlTokenStream, XmlBeanSerializerModifier, XmlBeanSerializerBase, XmlBeanPropertyWriter, XmlSerializationContext

Switch expressions/arrow notation — 11 conversions in 6 files

• FromXmlParser: 5 switches converted (arrow notation + switch expressions with yield)
• XmlTokenStream: 2 switches converted (switch expression + arrow notation with merged cases)
• XmlTokenBuffer: 1 switch expression with yield blocks
• XmlReadContext: 1 arrow notation conversion
• XmlTypeResolverBuilder + DefaultingXmlTypeResolverBuilder: 2 switch expressions folding empty defaults

Minor idioms — 16 edits in 10 files

• .length() == 0 → .isEmpty(): 6 locations
• String.format() → .formatted(): 4 locations
• Boolean null-guards → Boolean.TRUE.equals(): 3 locations
• Null ternaries → Objects.requireNonNullElse(): 3 locations

Verification: 609 tests pass, 0 failures.

[github-actions]

🧪 Code Coverage Report

Metric	Coverage	Change
Instructions	74.08%	📈 +0.030%
Branches	68.72%	📉 -0.110%

Coverage data generated from JaCoCo test results