Remove decommissioned test code

[matrei]

This pull request removes legacy Ant-based JUnit reporting support from the grails-test-core module. The main changes involve deleting custom JUnit report formatter classes and their factory, as well as cleaning up related dependencies from the build configuration.

Dependency cleanup:

• Removed Ant and JUnit-related dependencies from grails-test-core/build.gradle, including org.apache.ant:ant, org.apache.ant:ant-junit, and various JUnit and Spock dependencies that were only used by the deleted reporting classes. [1] [2]

Removal of legacy JUnit reporting implementation:

• Deleted the JUnitReports, JUnitReportsFactory, PlainFormatter, and XMLFormatter classes from grails-test-core/src/main/groovy/org/grails/test/report/junit/, which previously provided Ant-based JUnit report formatting and output. [1] [2] [3] [4]

[testlens-app]

✅ All tests passed ✅

🏷️ Commit: 8ac14e9
▶️ Tests: 53567 executed
⚪️ Checks: 60/60 completed

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[jdaugherty]

@matrei we removed support, but it's been requested we add junit support back via #15588 - aren't these needed for that?

[matrei]

@matrei we removed support, but it's been requested we add junit support back via #15588 - aren't these needed for that?

@jdaugherty As far as I understand, these are support classes for pre-Gradle test reporting.