SpringBoot Upgrade to Testcontainers 1.18.1

Comment From: wilkinsona

I'd like to revert this for now. It seems to have de-stabilised CI where we use RedpandaContainer, perhaps due to https://github.com/testcontainers/testcontainers-java/pull/6939. Speaking with @eddumelendez we may need to provide more resources to the CI workers. Until that's been done, I'd like to see if we can get snapshots publishing reliably again by downgrading to 1.18.0.

Comment From: wilkinsona

Superseded by https://github.com/spring-projects/spring-boot/issues/35677.