Declare `wiremock-jre8-standalone` in `micrometer-test` as optional

[mp911de]

When using micrometer-test as dependency, we get transitively wiremock-jre8-standalone that ships some bundled dependencies, such as SLF4J 1.7. It would be good to avoid a direct transitive dependency to avoid class path conflicts, especially given that SLF4J 2.0 is going to become the new target version.

[marcingrzejszczak]

We are using it for TCKs so it can't be optional.

BTW This is already reported in WireMock https://github.com/wiremock/wiremock/issues/1964