Upgrade minimum Java version to 17 and modernize code patterns

[marevol]

Summary

• Raise the minimum Java version from 11 to 17 and modernize codebase with Java 17 language features
• Bump project version to 3.1.0-SNAPSHOT to reflect the breaking baseline change

Changes Made

• Build & CI: Update Maven compiler release and source from 11 to 17; update GitHub Actions matrix to Java 17
• Version: Bump parent and module versions from 3.0.2-SNAPSHOT to 3.1.0-SNAPSHOT
• Pattern matching instanceof (JEP 394): Replace verbose cast-after-instanceof with pattern variables in Saml2Settings, SettingsBuilder
• Switch expressions (JEP 361): Convert traditional switch-case blocks to switch expressions in SchemaFactory
• Stream API: Replace .collect(Collectors.toList()) with .toList() in SettingsBuilder
• XPathFactory simplification: Remove JDK-specific XPathFactoryImpl workaround in Util, use standard XPathFactory.newInstance()
• Jakarta EE: Update sample web.xml namespace to Jakarta EE with web-app 6.1 schema
• Documentation: Update CLAUDE.md to reflect Java 17 baseline

Testing

• CI will run mvn -B package on Java 17/Ubuntu
• All existing tests should pass without modification

Breaking Changes

• Minimum Java version raised from 11 to 17 — applications using this library must run on Java 17+

Additional Notes

• No runtime behavior changes; all modifications are syntactic modernization or build configuration