Use already_configured_service in NWS strings

[MatthewFlamm]

Proposed change

Use already_configured_service in strings according to comment.

For #40578

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example entry for configuration.yaml:

# Example configuration.yaml

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue: Translations: Use references in config flow #40578
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
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[springstan]

Looks good!