Add default memory limit for shutdown-manager container

*Description*:

The shutdown-manager container has default resource requests but no resource limits.

This can cause issues in environments with strict resource policies or when a `LimitRange` is specified, which could inject a default limit that is not appropriate for this container. It also sets a good baseline for end-users, instead of having to gauge a proper limit.

Currently, users need to work around this by using the EnvoyProxy CRD's patch mechanism to inject limits, which is more cumbersome.

**Requested behavior**:

Add a default memory limit for the shutdown-manager container, similar to how the envoy container handles defaults.