SpringBoot Default value for NettyProperties.leakDetection is not aligned with Netty's default

As seen in #26958, the default value for the NettyProperties.leakDetection configuration property does not align with the default value in Netty's ResourceLeakDetector. We should ensure that this is always the case.

Comment From: wilkinsona

Re-opening as the test appears to be flakey. I think it's a test ordering problem.

Comment From: wilkinsona

4572ae5430120a7ff9493bdb89c264fd60f46a3a will hopefully stabilise the test.