update(repositories.yaml): promote plugin-sdk-cpp to incubating

[leogr]

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind bug

/kind cleanup

/kind documentation

/kind feature

/kind stable

/kind incubating

/kind sandbox

/kind deprecated

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area governance

/area proposals

/area utils

What this PR does / why we need it:

Dear @falcosecurity/core-maintainers

I'd like to promote https://github.com/falcosecurity/plugin-sdk-cpp to the Incubating level,

since it exited the experimental stage, has a release, and is actively maintained, but it also satisfies all criteria for the proposed level.

Thanks 🙏

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Once this is approved, I will take care of updating the project readme as well.

[poiana]

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