[chore] pin parent version

[vvcephei]

It looks like the last CP branch cut un-pinned the parent pom nanoversion. There is WIP to fix the tooling, so this is hopefully the last time a manual patch is needed.

7.6.0-139 is the latest nanoversion of common, so it's the version we have been building against for the last three days.

I have confirmed from reading the Jenkins logs that this is the version we used in the last successful build: https://github.com/confluentinc/ksql/runs/15183266336

[bideyio]

Thanks @vvcephei for taking care of this.

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