Update service domain for econet from 'water_heater' to 'econet'

[raman325]

Breaking Change:

This change breaks existing service call references to the water_heater.econet_* services by changing the service calls to be econet.*.

Description:

Update the domain and service name for econet.*. See this comment for context: #28890 (comment)

Related issue (if applicable): Related to #27289

Pull request with documentation for home-assistant.io (if applicable): N/A as the docs already mistakenly indicated that the services were in the econet domain

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly. Update and include derived files by running python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.