Bump opentelemetry-instrumentation-rdkafka to 0.4.2

[beccapearce]

Our builds are failing with the following error:

Your bundle is locked to opentelemetry-instrumentation-rdkafka (0.4.1) from
rubygems repository https://rubygems.org/ or installed locally, but that version
can no longer be found in that source. That means the author of
opentelemetry-instrumentation-rdkafka (0.4.1) has removed it. You'll need to
update your bundle to a version other than opentelemetry-instrumentation-rdkafka
(0.4.1) that hasn't been removed in order to install.

There seems to be an issue with the release of the previous gem so bumping it without relying on dependabot to get the builds working again.

An issue on open telemetry:
open-telemetry/opentelemetry-ruby-contrib#730