[Serverless]: Update naming across the board

[georgewallace]

Serverless Docs

Welcome to Elastic Serverless

Description

The official naming for serverless in general as well as the projects are listed below. We need to review our content to ensure we are using the right naming.

Serverless - Elastic Cloud Serverless
Elasticsearch - Elasticsearch Serverless
Observability - Elastic Observability Serverless
Security - Elastic Security Serverless

Resources and additional context

An example of something that needs to be updated can be seen below:

[leemthompo]

I updated variables that should address a lot of these, but I'm sure there'll be some hardcoded stragglers

[georgewallace]

variables to use:
@leemthompo can you confirm this is right based on the updates you are making?

Naming	When to use	variable
Elastic Cloud Serverless	when talking about serverless all up	{serverless-full}
Elasticsearch Serverless	when talking about the Elasticsearch project in serverless	{es-serverless}
Elasticsearch	when talking about Elasticsearch as a product	{es}

[leemthompo]

I confirm 👍

[leemthompo]

For those following along at home, this was the PR of interest: #196

[georgewallace]

The PR has been merged, this should be resolved.