Updated cmake_minimum_required to 3.15 to match Zeek 6.1.0

[javabrett]

Summary of the contribution

Updates cmake_minimum_required to 3.15. This is needed when building zeek-kafka in the zeek/zeek:6.1.0 (latest) Docker image, otherwise this compatibility error is raised:

CMake Error at /usr/local/zeek/share/zeek/cmake/ZeekPlugin.cmake:139 (message):
  Plugin requires CMake 3.0 which is less than Zeek's requirement (3.15.0).
  Please update cmake_minimum_required VERSION to 3.15 or higher.
Call Stack (most recent call first):
  CMakeLists.txt:20 (include)

This issue assumes that this change is ok to make for all supported Zeek versions, and that the plugin aspires to support latest Zeek 6.1.0 and beyond.

Testing

Built this Dockerfile:

FROM zeek/zeek:6.1.0

RUN apt-get update && \
    apt-get install -y cmake libssl-dev librdkafka-dev libpcap-dev g++

RUN zkg install --force https://github.com/javabrett/zeek-kafka.git --version cmake-version --user-var librdkafka_root=/usr

Checklist

• Confirm that any associated issues are indicated in the pull request title
• Rebase your PR against the latest commit within the target branch
• Include steps to verify and test the intended change
• Write or update unit tests and/or integration tests to verify your changes
• Run shellcheck against any new or updated shell scripts, indicating the reasoning behind any disabled checks
• Indicate whether or not this is a breaking change
• Ensure that any new dependencies are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0

[ottobackwards]

Thanks for the contribution, I pulled this into the 1.2.0 release which went out today, so you should be unblocked