Correct usage of term "ECID" / "Marketing Cloud Visitor ID"

Issue in ./help/dev/implement/server-side/sdk-guides/core-principles/user-identification-and-bucketing.md

If i'm not mistaken "Marketing Cloud Visitor ID (marketingCloudVisitorId)" now is called ECID mostly. I would fix that to reduce confusion when reading the docs.

Open ./help/dev/implement/server-side/sdk-guides/core-principles/user-identification-and-bucketing.md and review the references to “Marketing Cloud Visitor ID” and marketingCloudVisitorId. Update the terminology to reflect ECID where appropriate, while preserving the identifier name, then reread the section to confirm the wording is consistent.

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