update(repositories.yaml): demote to sandbox repos don't matching inc…

[leogr]

What type of PR is this?

/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:

Since the following repos do not match the criteria for the incubating status anymore

• https://github.com/falcosecurity/plugin-sdk-cpp has no releases and is not regularly updated
• https://github.com/falcosecurity/libs-sdk-go has no releases and is still experimental

This PR demotes them to sandbox.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

I've already opened the PRs to update the badges in those repositories:

• docs(README.md): add scope and status badges plugin-sdk-cpp#16
• docs(README.md): add scope and status badges libs-sdk-go#1

[maxgio92]

/approve

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: leogr, maxgio92

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• OWNERS [leogr,maxgio92]

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