feat(ElectricityMaps source): unify ElectricityMaps and ElectricityMapsFree

[unitrium]

Pull Request

Issue Number: https://github.com/Green-Software-Foundation/carbon-aware-sdk/issues/613

Summary

Remove all references to ElectricityMapsFree as it has been unified with ElectricityMaps.

Changes

• Update the documentation to remove references to ElectricityMapsFree and replace them when necessary with ElectricityMaps.
• Remove the controllers for ElectricityMapsFree and associated tests.
• Remove the free trial of ElectricityMaps mention as this has been replaced by a free permanent token.

Checklist

• [x] Local Tests Passing?
• [x] CICD and Pipeline Tests Passing?
• [ ] Added any new Tests?
• [x] Documentation Updates Made?
• [ ] Are there any API Changes? If yes, please describe below.
• [x] This is not a breaking change. If it is, please describe it below.

Are there API Changes?

No API changes expected.

Is this a breaking change?

The ElectricityMapsFree source will no longer be available.

Anything else?

cc @jbdietrich

Please follow GitHub's suggested syntax to link Pull Requests to Issues via keywords

This PR Closes #613