Fix some upcoming NullAway wildcard warnings

[msridhar]

We're starting to work on wildcard support in NullAway (see uber/NullAway#1520), and some warnings turned up when running that NullAway version on JUnit. This PR fixes the warnings. I think the fix makes sense: since ConfigurationParameters#get takes a Function<? super String, ? extends @Nullable T>, org.junit.jupiter.engine.config.JupiterConfiguration#getRawConfigurationParameter should take the same type for its Function parameter.

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Definition of Done

Most points N/A as this only changes annotations; no code behavior changes

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[testlens-app]

✅ All tests passed ✅

⚠️ TestLens detected flakiness ⚠️

Test Summary

Check	Project/Task	Test	Runs
CI / macOS	:jupiter-tests:test	SeparateThreadTimeoutInvocation > throws timeout exception when timeout duration is exceeded	🚫 ⚠️

🏷️ Commit: 1e12942
▶️ Tests: 17376 executed
⚪️ Checks: 15/15 completed

Test Failure

SeparateThreadTimeoutInvocation > throws timeout exception when timeout duration is exceeded (:jupiter-tests:test in CI / macOS)

java.lang.AssertionError: 
Expecting code to raise a throwable.
	at org.junit.jupiter.engine.extension.SeparateThreadTimeoutInvocationTests.throwsTimeoutException(SeparateThreadTimeoutInvocationTests.java:50)

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[mpkorstanje]

@msridhar I think you'll want to look at https://github.com/junit-team/junit-framework/actions/runs/24321136370/job/71047522373?pr=5602