Move the optional Jakarta integration app to a supported Spring Boot generation

[jmanico]

Follow-up to #90 (reviewed at 31588e1). This tracks work intentionally kept separate from the modernization PR.

Why

#90 moves jakarta-test/pom.xml to Boot 3.5.16 and Spring Framework 6.2.19 to address the identified dependency advisory while preserving the test app's existing Servlet 6 baseline.

Spring identifies 3.5.16 as the final OSS release of the 3.5 generation. A separate, deliberate major-version migration is needed for continued OSS updates. See the official release announcement.

Acceptance criteria

• Choose a supported Boot generation at implementation time and document its JDK, Servlet, JSP, EL, and container requirements.
• Upgrade the optional app and its test/container stack coherently using the appropriate dependency management.
• Retain the application-startup test and an actual Docker/Selenium test exercising the encoder's JSP tags and EL functions.
• Confirm the WAR packages and starts correctly and that documentation reflects how to run the tests locally and in CI.
• Keep Spring/Boot and newer container dependencies confined to the optional integration application, not the published encoder libraries.
• Do not silently raise the encoder libraries' Java or Jakarta Servlet compatibility baseline; preserve separate coverage for older supported consumers if the app now tests a newer container.
• Confirm the resulting dependency graph addresses the already identified Spring advisory and review newly selected dependencies.

This issue does not imply the published core encoder requires Spring Boot.