modernize build: jdk 17, github actions, license, tests

[crazysoftwarecoder]

Dusting off this one so it builds and runs on a current JDK.

• compile against JDK 17 (release 11); the project targeted Java 1.7 which modern javac no longer accepts
• replace javax.xml.bind.DatatypeConverter (removed in JDK 11) with java.util.Base64 in PageletBodyTag
• swap dead Travis CI for a GitHub Actions build workflow
• add an MIT LICENSE file and fix the readme (badge, license link pointed at someone else's repo, broken setter in the example, heading spacing)
• add unit tests for PageletTaskOutputHolder and the executor's task validation (8 tests)
• minor tidy: drop a leftover main(), rename isPageletTask since it returned the opposite

mvn clean verify passes locally on JDK 17.

[crazysoftwarecoder]

Reviewed the diff: no behavior changes to the runtime path beyond the Base64 swap (same output, just the supported API). Actions build is green and the 8 new tests pass. Merging.