Fix Dependabot configuration: remove unsupported cmake package ecosystem

[Copilot]

This PR fixes the Dependabot configuration error that was preventing dependency scanning from working properly.

Problem

Dependabot was failing to parse .github/dependabot.yml with the following error:

The property '#/updates/1/package-ecosystem' value "cmake" did not match one of the following values: 
npm, bundler, composer, devcontainers, dotnet-sdk, maven, mix, cargo, gradle, nuget, gomod, docker, 
docker-compose, elm, gitsubmodule, github-actions, pip, terraform, pub, rust-toolchain, swift, bun, 
uv, vcpkg, helm, conda

Root Cause

The configuration included package-ecosystem: "cmake" which is not a supported ecosystem by Dependabot. While this C++ project uses CMake for building, it manages dependencies through system packages via find_package() and pkg_check_modules() rather than a package manager that Dependabot can monitor.

Solution

• Removed the unsupported cmake package ecosystem entry
• Removed the docker package ecosystem entry (no Dockerfile exists in the repository)
• Kept the github-actions ecosystem entry for monitoring GitHub Actions workflow dependencies

Result

The Dependabot configuration now:

• ✅ Passes validation
• ✅ Will continue to monitor GitHub Actions dependencies for security updates
• ✅ Follows Dependabot's supported ecosystem specification

This maintains security monitoring for the parts of the project that Dependabot can actually scan while removing the invalid configurations that were causing parsing errors.

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