Remove unnecessary shutdown idle timeout configuration from JettyServletWebServerFactoryTests

[l2yujw]

This PR moves the Jetty connector shutdown idle timeout configuration from a
test-only customizer into main code when graceful shutdown is enabled.

Rationale

The Jetty servlet web server tests currently set shutdownIdleTimeout via a
customizer and include a TODO to configure it in main code. Keeping this as a
test-only workaround can lead to inconsistent behavior for users relying on
graceful shutdown.

Changes

• Apply shutdownIdleTimeout to Jetty ServerConnectors when Shutdown.GRACEFUL is enabled
• Remove the test customizer and the related TODO

Testing

• ./gradlew :module:spring-boot-jetty:test

[wilkinsona]

Thanks for the PR. I wrote the original TODO but, unfortunately, I didn't keep good enough notes as to why it was necessary. Having just experimented a little, it doesn't seem to be necessary any more as all tests in JettyServletWebServerFactoryTests pass without it. Rather than changing main code, would you like to update your PR to only change the tests and remove the idle timeout configuration?

[l2yujw]

Thanks for confirming. Updated as suggested: I removed the main code change and kept the PR focused on the tests by removing the idle timeout configuration (and the related TODO). All tests in JettyServletWebServerFactoryTests still pass.

[wilkinsona]

Thanks very much, @l2yujw.