docs: Remove secret limit note for service migration

[akim-newrelic]

Description

Once we switch nerd-storage-vault to use Customer Secrets Service instead of Synthetics Locker, the secret limit per actor will no longer apply. This PR removes that note from the doc, and should be merged after the switch is made.

Related Issue(s) / Ticket(s)

https://new-relic.atlassian.net/browse/NR-267056

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[adutta-newrelic]

Closing this PR, as the changes has already been madde and approved in https://github.com/newrelic/docs-website/pull/20275