Add Serenita SaaS

[eth2353]

Description

Adds Serenita (https://serenita.io) to list of staking service providers.

Related Issue

Resolves #15814

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[github-actions]

This issue is stale because it has been open 30 days with no activity.

[wackerow]

Hey @eth2353! Sorry for delays here, but this looks good, great to see the progress here... Pulling in!

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